Don't DLL-export the private wxSimpleFontEnumerator class.

This class is only used internally, so there is no need to export it from the
shared library.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76903 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775