block: add blk_set_queue_dying() to blkdev.h

We export this function and NVMe wants to use it, but for some reason
it was never added to the block header. Do that.
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/blkdev.h

@@ -1024,6 +1024,7 @@ bool __must_check blk_get_queue(struct request_queue *);
 struct request_queue *blk_alloc_queue(gfp_t);
 struct request_queue *blk_alloc_queue_node(gfp_t, int);
 extern void blk_put_queue(struct request_queue *);
+extern void blk_set_queue_dying(struct request_queue *);
 
 /*
  * block layer runtime pm functions