Fix missing use in mod/ping

mod/ping.php

@@ -9,6 +9,7 @@ use Friendica\Content\ForumManager;
 use Friendica\Content\Text\BBCode;
 use Friendica\Core\Addon;
 use Friendica\Core\Cache;
+use Friendica\Core\Config;
 use Friendica\Core\L10n;
 use Friendica\Core\PConfig;
 use Friendica\Core\System;