dnd: Declare restore location variables

https://bugzilla.gnome.org/show_bug.cgi?id=791233

js/ui/dnd.js

@@ -583,7 +583,7 @@ var _Draggable = new Lang.Class({
 
     _restoreDragActor: function(eventTime) {
         this._dragInProgress = false;
-        [restoreX, restoreY, restoreScale] = this._getRestoreLocation();
+        let [restoreX, restoreY, restoreScale] = this._getRestoreLocation();
 
         // fade the actor back in at its original location
         this._dragActor.set_position(restoreX, restoreY);