dnd: Declare dragMonitors with var

It is used outside the DND module itself.

https://bugzilla.gnome.org/show_bug.cgi?id=787907

js/ui/dnd.js

@@ -38,10 +38,10 @@ var DragDropResult = {
     SUCCESS:  1,
     CONTINUE: 2
 };
+var dragMonitors = [];
 
 let eventHandlerActor = null;
 let currentDraggable = null;
-let dragMonitors = [];
 
 function _getEventHandlerActor() {
     if (!eventHandlerActor) {