Backends: Win32: avoid skipping DefWindowProc() on WM_CLOSE.

Mostly because I don't have a trace of why I did it this way, even in viewport branch history.
diff --git a/backends/imgui_impl_win32.cpp b/backends/imgui_impl_win32.cpp
index c079a2b..63b57de 100644
--- a/backends/imgui_impl_win32.cpp
+++ b/backends/imgui_impl_win32.cpp
@@ -1265,7 +1265,7 @@
         {
         case WM_CLOSE:
             viewport->PlatformRequestClose = true;
-            return 0;
+            break;
         case WM_MOVE:
             viewport->PlatformRequestMove = true;
             break;