Add missing requires

Without explicit requires, it is possible that a build would exclude ol.js.

src/ol/source/bingmapssource.js

@@ -4,6 +4,7 @@ goog.require('goog.Uri');
 goog.require('goog.array');
 goog.require('goog.asserts');
 goog.require('goog.net.Jsonp');
+goog.require('ol');
 goog.require('ol.Attribution');
 goog.require('ol.TileRange');
 goog.require('ol.TileUrlFunction');