t10-pi: provide empty t10_pi_complete() for !CONFIG_BLK_DEV_INTEGRITY

Fixes a link failure whtn BLK_DEV_INTEGRITY isn't defined.

Fixes: 10c41ddd

 ("block: move dif_prepare/dif_complete functions to block layer")
Signed-off-by: Jens Axboe <axboe@kernel.dk>