workqueue: move struct worker definition to workqueue_internal.h

This will be used to implement an inline function to query whether
%current is a workqueue worker and, if so, allow determining which
work item it's executing.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>

diff --cc kernel/workqueue.c
Simple merge

diff --cc kernel/workqueue_internal.h
Simple merge