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/generic/graphicc.cpp

@@ -41,7 +41,7 @@ bool wxCairoInit();
 #include "wx/private/graphics.h"
 #include "wx/rawbmp.h"
 #include "wx/vector.h"
-#if defined(__WXMSW__) && wxUSE_ENH_METAFILE
+#ifdef __WXMSW__
     #include "wx/msw/enhmeta.h"
 #endif