block: don't include blk-mq.h in blk.h

No needed, shift a blk-stat.h include into the source file that needs it
instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211123185312.1432157-6-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>