workqueue: declare system_highpri_wq

system_highpri_wq is exported to modules via EXPORT_SYMBOL_GPL(),
but it was forgotten to be declared in workqueue.h. So we add the declaration
and a short description for it.

tj: Minor comment tweak.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>