MAINTAINERS: Add workqueue_internal.h to the WORKQUEUE entry

This file doesn't have a maintainer. It should belong to the WORKQUEUE
entry part. Add it to the WORKQUEUE entry.

Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>