WorkspaceThumbnail: fix mutter warning

Pass a proper timestamp to workspace activation functions

https://bugzilla.gnome.org/show_bug.cgi?id=694365

js/ui/workspaceThumbnail.js

@@ -590,7 +590,7 @@ const ThumbnailsBox = new Lang.Class({
             let thumbnail = this._thumbnails[i]
             let [w, h] = thumbnail.actor.get_transformed_size();
             if (y >= thumbnail.actor.y && y <= thumbnail.actor.y + h) {
-                thumbnail.activate(event.time);
+                thumbnail.activate(event.get_time());
                 break;
             }
         }