This patch avoids that building with W=1 C=2 triggers the following
warning:

block/bio-integrity.c:35:6: warning: symbol 'blk_flush_integrity' was not declared. Should it be static?
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Jens Axboe <axboe@fb.com>