MDL-39087 Fix missing cronlib inclusion in file_storage::cron()

This is not actually related to MDL-38259 but it was discovered while
running unit tests for file_storage. When running the tests for this
class separately, the cronlib.php was not included (it is included
when this method is normally called during cron execution).

lib/filestorage/file_storage.php

@@ -2015,6 +2015,7 @@ class file_storage {
      */
     public function cron() {
         global $CFG, $DB;
+        require_once($CFG->libdir.'/cronlib.php');
 
         // find out all stale draft areas (older than 4 days) and purge them
         // those are identified by time stamp of the /. root dir