Add required use statement in mod/subthread

mod/subthread.php

@@ -6,6 +6,7 @@ use Friendica\App;
 use Friendica\Core\Hook;
 use Friendica\Core\L10n;
 use Friendica\Core\Logger;
+use Friendica\Core\Session;
 use Friendica\Core\System;
 use Friendica\Database\DBA;
 use Friendica\Model\Item;