Examples: Add missing include guards (#2166)

diff --git a/examples/imgui_impl_dx10.h b/examples/imgui_impl_dx10.h
index 263073e..6e0ff4a 100644
--- a/examples/imgui_impl_dx10.h
+++ b/examples/imgui_impl_dx10.h
@@ -8,6 +8,8 @@
 // If you are new to dear imgui, read examples/README.txt and read the documentation at the top of imgui.cpp.
 // https://github.com/ocornut/imgui
 
+#pragma once
+
 struct ID3D10Device;
 
 IMGUI_IMPL_API bool     ImGui_ImplDX10_Init(ID3D10Device* device);
diff --git a/examples/imgui_impl_dx11.h b/examples/imgui_impl_dx11.h
index 724b469..8eda59f 100644
--- a/examples/imgui_impl_dx11.h
+++ b/examples/imgui_impl_dx11.h
@@ -8,6 +8,8 @@
 // If you are new to dear imgui, read examples/README.txt and read the documentation at the top of imgui.cpp.
 // https://github.com/ocornut/imgui
 
+#pragma once
+
 struct ID3D11Device;
 struct ID3D11DeviceContext;
 
diff --git a/examples/imgui_impl_dx12.h b/examples/imgui_impl_dx12.h
index 8c46254..7ead88e 100644
--- a/examples/imgui_impl_dx12.h
+++ b/examples/imgui_impl_dx12.h
@@ -10,6 +10,8 @@
 // If you are new to dear imgui, read examples/README.txt and read the documentation at the top of imgui.cpp.
 // https://github.com/ocornut/imgui
 
+#pragma once
+
 enum DXGI_FORMAT;
 struct ID3D12Device;
 struct ID3D12GraphicsCommandList;
diff --git a/examples/imgui_impl_dx9.h b/examples/imgui_impl_dx9.h
index 03da1bd..95902f7 100644
--- a/examples/imgui_impl_dx9.h
+++ b/examples/imgui_impl_dx9.h
@@ -8,6 +8,8 @@
 // If you are new to dear imgui, read examples/README.txt and read the documentation at the top of imgui.cpp.
 // https://github.com/ocornut/imgui
 
+#pragma once
+
 struct IDirect3DDevice9;
 
 IMGUI_IMPL_API bool     ImGui_ImplDX9_Init(IDirect3DDevice9* device);
diff --git a/examples/imgui_impl_freeglut.h b/examples/imgui_impl_freeglut.h
index 6289ed4..909f072 100644
--- a/examples/imgui_impl_freeglut.h
+++ b/examples/imgui_impl_freeglut.h
@@ -9,6 +9,8 @@
 // If you are new to dear imgui, read examples/README.txt and read the documentation at the top of imgui.cpp.
 // https://github.com/ocornut/imgui
 
+#pragma once
+
 IMGUI_IMPL_API bool     ImGui_ImplFreeGLUT_Init();
 IMGUI_IMPL_API void     ImGui_ImplFreeGLUT_InstallFuncs();
 IMGUI_IMPL_API void     ImGui_ImplFreeGLUT_Shutdown();
diff --git a/examples/imgui_impl_glfw.h b/examples/imgui_impl_glfw.h
index a9bab99..938fe78 100644
--- a/examples/imgui_impl_glfw.h
+++ b/examples/imgui_impl_glfw.h
@@ -16,6 +16,8 @@
 // The 'glsl_version' initialization parameter defaults to "#version 150" if NULL.
 // Only override if your GL version doesn't handle this GLSL version. Keep NULL if unsure!
 
+#pragma once
+
 struct GLFWwindow;
 
 IMGUI_IMPL_API bool     ImGui_ImplGlfw_InitForOpenGL(GLFWwindow* window, bool install_callbacks);
diff --git a/examples/imgui_impl_marmalade.h b/examples/imgui_impl_marmalade.h
index 96c4333..8a767f9 100644
--- a/examples/imgui_impl_marmalade.h
+++ b/examples/imgui_impl_marmalade.h
@@ -8,6 +8,8 @@
 // If you are new to dear imgui, read examples/README.txt and read the documentation at the top of imgui.cpp.
 // https://github.com/ocornut/imgui
 
+#pragma once
+
 IMGUI_IMPL_API bool     ImGui_Marmalade_Init(bool install_callbacks);
 IMGUI_IMPL_API void     ImGui_Marmalade_Shutdown();
 IMGUI_IMPL_API void     ImGui_Marmalade_NewFrame();
diff --git a/examples/imgui_impl_opengl2.h b/examples/imgui_impl_opengl2.h
index 911447a..e901cb7 100644
--- a/examples/imgui_impl_opengl2.h
+++ b/examples/imgui_impl_opengl2.h
@@ -16,6 +16,8 @@
 // confuse your GPU driver. 
 // The GL2 code is unable to reset attributes or even call e.g. "glUseProgram(0)" because they don't exist in that API.
 
+#pragma once
+
 IMGUI_IMPL_API bool     ImGui_ImplOpenGL2_Init();
 IMGUI_IMPL_API void     ImGui_ImplOpenGL2_Shutdown();
 IMGUI_IMPL_API void     ImGui_ImplOpenGL2_NewFrame();
diff --git a/examples/imgui_impl_opengl3.h b/examples/imgui_impl_opengl3.h
index 4c3321f..fba03d0 100644
--- a/examples/imgui_impl_opengl3.h
+++ b/examples/imgui_impl_opengl3.h
@@ -19,6 +19,8 @@
 // On computer platform the GLSL version default to "#version 130". On OpenGL ES 3 platform it defaults to "#version 300 es"
 // Only override if your GL version doesn't handle this GLSL version. See GLSL version table at the top of imgui_impl_opengl3.cpp.
 
+#pragma once
+
 // Set default OpenGL loader to be gl3w
 #if !defined(IMGUI_IMPL_OPENGL_LOADER_GL3W)     \
  && !defined(IMGUI_IMPL_OPENGL_LOADER_GLEW)     \
diff --git a/examples/imgui_impl_sdl.h b/examples/imgui_impl_sdl.h
index 7fb2b86..124d704 100644
--- a/examples/imgui_impl_sdl.h
+++ b/examples/imgui_impl_sdl.h
@@ -14,6 +14,8 @@
 // If you are new to dear imgui, read examples/README.txt and read the documentation at the top of imgui.cpp.
 // https://github.com/ocornut/imgui
 
+#pragma once
+
 struct SDL_Window;
 typedef union SDL_Event SDL_Event;
 
diff --git a/examples/imgui_impl_vulkan.h b/examples/imgui_impl_vulkan.h
index ca0c2b5..eda25ef 100644
--- a/examples/imgui_impl_vulkan.h
+++ b/examples/imgui_impl_vulkan.h
@@ -11,6 +11,8 @@
 // The aim of imgui_impl_vulkan.h/.cpp is to be usable in your engine without any modification. 
 // IF YOU FEEL YOU NEED TO MAKE ANY CHANGE TO THIS CODE, please share them and your feedback at https://github.com/ocornut/imgui/
 
+#pragma once
+
 #include <vulkan/vulkan.h>
 
 #define IMGUI_VK_QUEUED_FRAMES      2
diff --git a/examples/imgui_impl_win32.h b/examples/imgui_impl_win32.h
index ed4f419..0f2aaa2 100644
--- a/examples/imgui_impl_win32.h
+++ b/examples/imgui_impl_win32.h
@@ -8,6 +8,8 @@
 // Missing features:
 //  [ ] Platform: Gamepad support (best leaving it to user application to fill io.NavInputs[] with gamepad inputs from their source of choice).
 
+#pragma once
+
 IMGUI_IMPL_API bool     ImGui_ImplWin32_Init(void* hwnd);
 IMGUI_IMPL_API void     ImGui_ImplWin32_Shutdown();
 IMGUI_IMPL_API void     ImGui_ImplWin32_NewFrame();