block: Expose kblock_schedule_delayed_work()

This function is exported in blk-core.c to be used in other modules
but it's definition in h file is missing.

Change-Id: I3374d97774adf06c1f93e40f1d5e1daefb983571
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>