fix missing global (how did this pass tests? o.o)

ext/ban_words/main.php

@@ -54,6 +54,7 @@ xanax
 	}
 
 	public function onCommentPosting(CommentPostingEvent $event) {
+		global $user;
 		if(!$user->can("bypass_comment_checks")) {
 			$this->test_text($event->comment, new CommentPostingException("Comment contains banned terms"));
 		}