Allow imgui_internal.h to be included before imgui.h

(practically speaking it allows me to use StructLayout without hassle)
diff --git a/imgui_internal.h b/imgui_internal.h
index 6681f07..0e813ef 100644
--- a/imgui_internal.h
+++ b/imgui_internal.h
@@ -43,7 +43,7 @@
 //-----------------------------------------------------------------------------
 
 #ifndef IMGUI_VERSION
-#error Must include imgui.h before imgui_internal.h
+#include "imgui.h"
 #endif
 
 #include <stdio.h>      // FILE*, sscanf