ui: restrict gtk to only use the x11 backend

We use GTK as a way to get the X11 connection for our internal
use, so we need it to keep using X.

https://bugzilla.gnome.org/show_bug.cgi?id=706303

src/ui/ui.c

@@ -56,6 +56,8 @@ struct _MetaUI
 void
 meta_ui_init (void)
 {
+  gdk_set_allowed_backends ("x11");
+
   if (!gtk_init_check (NULL, NULL))
     meta_fatal ("Unable to open X display %s\n", XDisplayName (NULL));
 }