don't include wx/msw/winundef.h when including wx/defs.h from C code (closes #10910)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@61238 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/defs.h

@@ -2855,9 +2855,11 @@ typedef const void* WXWidget;
 /*  This is required because of clashing macros in windows.h, which may be */
 /*  included before or after wxWidgets classes, and therefore must be */
 /*  disabled here before any significant wxWidgets headers are included. */
+#ifdef __cplusplus
 #ifdef __WXMSW__
 #include "wx/msw/winundef.h"
 #endif /*  __WXMSW__ */
+#endif /* __cplusplus */
 
 /*  --------------------------------------------------------------------------- */
 /*  macro to define a class without copy ctor nor assignment operator */