block: make blk_timeout_init() static

The sparse tool complains as follows:

block/blk-timeout.c:93:12: warning:
 symbol 'blk_timeout_init' was not declared. Should it be static?

Function blk_timeout_init() is not used outside of blk-timeout.c, so
mark it static.

Fixes: 9054650f ("block: relax jiffies rounding for timeouts")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>