Make wxEnhMetaFileDC ctor from wxDC explicit.

This avoids implicit conversion of any kind of wxDC to wxEnhMetaFileDC which
is totally unexpected.

See #13326.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68317 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/enhmeta.h

@@ -89,6 +89,7 @@ public:
 
     // as above, but takes reference DC as first argument to take resolution,
     // size, font metrics etc. from
+    wxEXPLICIT
     wxEnhMetaFileDC(const wxDC& referenceDC,
                     const wxString& filename = wxEmptyString,
                     int width = 0, int height = 0,