Fix missing `use Piwik\Piwik`

See https://builds-artifacts.piwik.org/piwik/piwik/master/17471/processed/Login_formless_login.png

plugins/Login/Auth.php

@@ -11,6 +11,7 @@ namespace Piwik\Plugins\Login;
 use Exception;
 use Piwik\AuthResult;
 use Piwik\Db;
+use Piwik\Piwik;
 use Piwik\Plugins\UsersManager\Model;
 use Piwik\Plugins\UsersManager\UsersManager;
 use Piwik\Session;