Add missing use statement.

Fixes #3776

src/Controller/Component/AuthComponent.php

@@ -15,6 +15,7 @@
 namespace Cake\Controller\Component;
 
 use Cake\Controller\Component;
+use Cake\Controller\ComponentRegistry;
 use Cake\Controller\Controller;
 use Cake\Core\App;
 use Cake\Core\Configure;