prevent deprecated warning, fixes #1818

Closes #1818

Merge request studip/studip!1195

lib/models/StatusgruppeUser.php

@@ -68,7 +68,7 @@ class StatusgruppeUser extends SimpleORMap implements PrivacyObject
      * @param Contact $contact
      * @return StatusgruppeUser[]
      */
-    public function findByContact(Contact $contact)
+    public static function findByContact(Contact $contact)
     {
         return self::findBySQL("INNER JOIN `statusgruppen` USING(`statusgruppe_id`) WHERE `statusgruppen`.`range_id` = ? AND `statusgruppe_user`.`user_id` = ?", [$contact->owner_id, $contact->user_id]);
     }