Add missing NOTIFICATION_CONFIRM_SUBMAIL_FAIL lang constant

PRevents a PHP 8 compatibility error when subscribing to a comment thread failed

lang/serendipity_lang_pt.inc.php

@@ -1282,3 +1282,4 @@
 @define('PLUGIN_GROUP_ALL', 'All categories');
 @define('PENDING_COMMENT_SUBSCRIPTION', 'Pending confirmation');
 @define('NO_COMMENT_SUBSCRIPTION', 'Not subscribed');
+@define('NOTIFICATION_CONFIRM_SUBMAIL_FAIL', 'Sorry, the confirmation of your subscription to new comments has failed.');