minor #14564 [Security] Add forgotten use (dvigo)

This PR was submitted for the 5.x branch but it was merged into the 4.4 branch instead.

Discussion
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[Security] Add forgotten use

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Commits
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ad7a7ce Add forgotten use

Author: javiereguiluz

security/access_denied_handler.rst

@@ -29,6 +29,7 @@ unauthenticated user tries to access a protected resource::
     use Symfony\Component\HttpFoundation\RedirectResponse;
     use Symfony\Component\HttpFoundation\Request;
     use Symfony\Component\HttpFoundation\Session\SessionInterface;
+    use Symfony\Component\Routing\Generator\UrlGeneratorInterface;
     use Symfony\Component\Security\Core\Exception\AuthenticationException;
     use Symfony\Component\Security\Http\EntryPoint\AuthenticationEntryPointInterface;