MAINTAINERS: Add workqueue entry

Signed-off-by: Tejun Heo <[e-mail]>

MAINTAINERS

···
 F:	include/sound/wm????.h
 F:	sound/soc/codecs/wm*
 
+WORKQUEUE
+M:	Tejun Heo <[e-mail]>
+L:	[e-mail]
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git
+S:	Maintained
+F:	include/linux/workqueue.h
+F:	kernel/workqueue.c
+F:	Documentation/workqueue.txt
+
 X.25 NETWORK LAYER
 M:	Andrew Hendry <[e-mail]>
 L:	[e-mail]