Fix Call to a member function getCacheTime() on a non-object in RefreshLinksJob.php

PHP Fatal error:  Call to a member function getCacheTime() on a non-object
in /www/translatewiki.net/w/includes/job/jobs/RefreshLinksJob.php on line 144

Change-Id: Ie30956823c8537bb2d459062c7719d1a59e26867