Don't notify continuously when MySQL slave SQL thread is running

I'm getting notified continuously when MySQL is correctly running as a slave:

    [2013-05-29 14:22:19] Notification: severity = OKAY, host = eu1.thumbr.it, plugin = mysql, plugin_instance = thumbrit, type = time_offset, message = slave SQL thread started
    [2013-05-29 14:22:29] Notification: severity = OKAY, host = eu1.thumbr.it, plugin = mysql, plugin_instance = thumbrit, type = time_offset, message = slave SQL thread started
    [2013-05-29 14:22:39] Notification: severity = OKAY, host = eu1.thumbr.it, plugin = mysql, plugin_instance = thumbrit, type = time_offset, message = slave SQL thread started
    [2013-05-29 14:22:49] Notification: severity = OKAY, host = eu1.thumbr.it, plugin = mysql, plugin_instance = thumbrit, type = time_offset, message = slave SQL thread started

I think this small patch fixes the problem. I didn't tested it yet.