BLOCK: Hide the contents of linux/bio.h if CONFIG_BLOCK=n

Hide the contents of linux/bio.h if CONFIG_BLOCK=n as there shouldn't be
compiled code that uses it.

Signed-off-by: David Howells <dhowells@redhat.com>
Cc: James Bottomley <James.Bottomley@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>