Remove unnecessary _mingw.h inclusion from wx/msw/missing.h.

This header is already included from wx/platform.h which is always included
before wx/msw/missing.h.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75556 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/missing.h

@@ -659,11 +659,6 @@ typedef struct
 
 //We need to check if we are using MinGW or mingw-w64 as their
 //definitions are different
-
-#ifdef __MINGW32__
-#include <_mingw.h>
-#endif
-
 #ifdef __MINGW32_TOOLCHAIN__
 typedef enum CommandStateChangeConstants {
     CSC_UPDATECOMMANDS = (int) 0xFFFFFFFF,