Setup bitmaps for all menu item kinds in wxGTK

This doesn't change anything for now, but doesn't cost anything and
might avoid problems in the future if another menu item kind is added.

src/gtk/menu.cpp

@@ -1147,7 +1147,7 @@ void wxMenu::SetupBitmaps(wxWindow *win)
         wxMenuItem *menuitem = node->GetData();
         if (menuitem->IsSubMenu())
             menuitem->GetSubMenu()->SetupBitmaps(win);
-        if (menuitem->GetKind() == wxITEM_NORMAL)
+        if (!menuitem->IsSeparator())
             menuitem->SetupBitmaps(win);
         node = node->GetNext();
     }