Added missing `use` statement (again), thanks to @MrPetovan

Yes, we need more unit-testing! Not only to rely on your IDE that it will locate
it for you.

mod/oexchange.php

@@ -21,6 +21,7 @@
 
 use Friendica\App;
 use Friendica\Content\Text\BBCode;
+use Friendica\Content\Text\HTML;
 use Friendica\Core\System;
 use Friendica\DI;
 use Friendica\Module\Response;