Better type definition

Fixes the `Invalid type(s) for operator IN.` warnings from the closure compiler.

src/ol/control/Attribution.js

@@ -130,7 +130,7 @@ inherits(Attribution, Control);
 Attribution.prototype.getSourceAttributions_ = function(frameState) {
   /**
    * Used to determine if an attribution already exists.
-   * @type {Object.<string, boolean>}
+   * @type {!Object.<string, boolean>}
    */
   const lookup = {};