updated plugin to use new logout.php

lib/classes/auth_plugins/StudipAuthSimpleSamlPHP.php

@@ -100,7 +100,7 @@ class StudipAuthSimpleSamlPHP extends StudipAuthSSO
     /**
      * Logout the user.
      */
-    public function logout()
+    public function logout(): void
     {
         $auth = new \SimpleSAML\Auth\Simple($this->sp_name);
         $auth->Logout();