[feature/system-cron] Require cron task wrapper constructor to be a cron task.

PHPBB3-9596

phpBB/includes/cron/cron_task_wrapper.php

@@ -33,7 +33,7 @@ class cron_task_wrapper
 	/**
 	* Wraps a task $task, which must implement cron_task interface.
 	*/
-	public function __construct($task)
+	public function __construct(cron_task $task)
 	{
 		$this->task = $task;
 	}