Fix regrabbing of keys on layout change

In commit 532ec0cd, we started grabbing keys on the client window directly
instead of its parent window (the frame window). This commit fixes one place
which was overlooked back then.

Signed-off-by: Uli Schlachter <psychon@znc.in>

event.c

@@ -856,7 +856,7 @@ event_handle_mappingnotify(xcb_mapping_notify_event_t *ev)
         foreach(_c, globalconf.clients)
         {
             client_t *c = *_c;
-            xwindow_grabkeys(c->frame_window, &c->keys);
+            xwindow_grabkeys(c->window, &c->keys);
         }
     }
 }