Importing missing namespaces.

Author: dlondero

cookbook/security/acl.rst

@@ -84,7 +84,12 @@ Creating an ACL, and adding an ACE
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 .. code-block:: php
-
+    use Symfony\Component\Security\Core\Exception\AccessDeniedException
+    use Symfony\Component\Security\Acl\Domain\ObjectIdentity;
+    use Symfony\Component\Security\Acl\Domain\UserSecurityIdentity;
+    use Symfony\Component\Security\Acl\Permission\MaskBuilder;
+    // ...
+    
     // BlogController.php
     public function addCommentAction(Post $post)
     {