block: remove export for blk_queue_bio

With commit ff36ab34 ("dm: remove request-based logic from
make_request_fn wrapper") DM no longer calls blk_queue_bio() directly,
so remove its export.  Doing so required a forward declaration in
blk-core.c.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>