Move wxUSE_ENH_METAFILE check inside wx/msw/enhmeta.h itself

This is more consistent with the other headers, which are safe to
include even when the corresponding feature is turned off.

Also remove the now redundant wxUSE_ENH_METAFILE checks in the source
code including this header.

See https://github.com/wxWidgets/wxWidgets/pull/951

src/common/graphcmn.cpp

@@ -32,10 +32,8 @@
 #endif
 
 #ifdef __WXMSW__
-#if wxUSE_ENH_METAFILE
     #include "wx/msw/enhmeta.h"
 #endif
-#endif
 
 #include "wx/private/graphics.h"