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>

MAINTAINERS

@@ -22596,6 +22596,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git
 F:	Documentation/core-api/workqueue.rst
 F:	include/linux/workqueue.h
 F:	kernel/workqueue.c
+F:	kernel/workqueue_internal.h
 
 WWAN DRIVERS
 M:	Loic Poulain <loic.poulain@linaro.org>