MAINTAINERS: add bio.h to the block section

bio.h is part of the block layer, so list it in the MAINTAINERS file
as such.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220127064221.1314477-1-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

MAINTAINERS

@@ -3436,6 +3436,7 @@ F:	Documentation/ABI/stable/sysfs-block
 F:	Documentation/block/
 F:	block/
 F:	drivers/block/
+F:	include/linux/bio.h
 F:	include/linux/blk*
 F:	kernel/trace/blktrace.c
 F:	lib/sbitmap.c