Add missing @api

These types are referenced in the externs but were not declared.
Found by ol3-cesium.

src/ol/loadingstrategy.js

@@ -6,6 +6,7 @@ goog.require('ol.TileCoord');
 
 /**
  * @typedef {function(ol.Extent, number): Array.<ol.Extent>}
+ * @api
  */
 ol.LoadingStrategy;