backends/x11: Fix memory leak in meta_x11_handle_event()

The event was leaked after it had been processed.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2192>

src/backends/x11/meta-event-x11.c

@@ -103,6 +103,7 @@ meta_x11_handle_event (XEvent *xevent)
     {
       /* forward the event into clutter for emission etc. */
       clutter_do_event (event);
+      clutter_event_free (event);
       --spin;
     }