Don't duplicate INVALID_FILE_ATTRIBUTES definition.

Move it in wx/msw/missing.h header instead of defining it in two different
places.

Closes #12964.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66996 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/missing.h

@@ -537,5 +537,9 @@ typedef struct
     #endif
 #endif  // defined __WINE__
 
+#ifndef INVALID_FILE_ATTRIBUTES
+    #define INVALID_FILE_ATTRIBUTES ((DWORD)-1)
+#endif
+
 #endif
     // _WX_MISSING_H_