Add missing wxUSE_MENUS checks to generic MDI classes

Make mdig.cpp compile with wxUSE_MENUS==0, as it already had many
wxUSE_MENUS checks and only missed a few more of them.

See #9626.

include/wx/generic/mdig.h

@@ -199,7 +199,9 @@ protected:
     void Init();
 
 private:
+#if wxUSE_MENUS
     void OnMenuHighlight(wxMenuEvent& event);
+#endif // wxUSE_MENUS
     void OnClose(wxCloseEvent& event);
 
     wxDECLARE_DYNAMIC_CLASS(wxGenericMDIChildFrame);