xen-blkfront.c: make blkif_ioctl() static

This patch makes the needlessly global blkif_ioctl() static.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>