Disable wxUSE_ENH_METAFILE for wxGTK builds.

This can happen to be defined in wxGTK builds under Windows.

Closes #15332.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74517 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775