workqueue: use kernel.h for workqueue header file

Workqueues will be brought in as a first-class unified kernel object,
not a misc functionality. Do not use the contents of the header file
when building a unified kernel, but kernel.h instead.

Change-Id: I649558fee92b6565ada0eee81bde9f542a468f9f
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>