block: add wbt_disable_default export for BFQ

This isn't unused, if BFQ is modular we get into trouble.

Fixes: b6676f65 ("block: remove a few unused exports")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-wbt.c

@@ -754,6 +754,7 @@ void wbt_disable_default(struct request_queue *q)
 	if (rwb->enable_state == WBT_STATE_ON_DEFAULT)
 		rwb->wb_normal = 0;
 }
+EXPORT_SYMBOL_GPL(wbt_disable_default);
 
 static struct rq_qos_ops wbt_rqos_ops = {
 	.throttle = wbt_wait,