[feature/system-cron] Added includes for class dependencies.

PHPBB3-9596

phpBB/includes/cron/cron_task_base.php

@@ -16,6 +16,8 @@ if (!defined('IN_PHPBB'))
 	exit;
 }
 
+include_once($phpbb_root_path . 'includes/cron/cron_task.' . $phpEx);
+
 /**
 * Cron task base class. Provides sensible defaults for cron tasks
 * and partially implements cron task interface, making writing cron tasks easier.