Add missing use statement in mod/community

- Addresses https://github.com/friendica/friendica/issues/7998#issuecomment-569265877

mod/community.php

@@ -13,6 +13,7 @@ use Friendica\Core\Config;
 use Friendica\Core\L10n;
 use Friendica\Core\PConfig;
 use Friendica\Core\Renderer;
+use Friendica\Core\Session;
 use Friendica\Database\DBA;
 use Friendica\Model\Item;
 use Friendica\Model\User;