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

include/wx/msw/enhmeta.h

@@ -11,6 +11,10 @@
 #ifndef _WX_MSW_ENHMETA_H_
 #define _WX_MSW_ENHMETA_H_
 
+#include "wx/defs.h"
+
+#if wxUSE_ENH_METAFILE
+
 #include "wx/dc.h"
 #include "wx/gdiobj.h"
 
@@ -192,4 +196,6 @@ protected:
 
 #endif // wxUSE_DATAOBJ
 
+#endif // wxUSE_ENH_METAFILE
+
 #endif // _WX_MSW_ENHMETA_H_