block: add __init to blkcg_policy_register

blkcg_policy_register is only called by
__init functions:

__init cfq_init
__init throtl_init

Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Jens Axboe <axboe@fb.com>