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>

include/linux/workqueue.h

@@ -340,6 +340,9 @@ enum {
  * short queue flush time.  Don't queue works which can run for too
  * long.
  *
+ * system_highpri_wq is similar to system_wq but for work items which
+ * require WQ_HIGHPRI.
+ *
  * system_long_wq is similar to system_wq but may host long running
  * works.  Queue flushing might take relatively long.
  *
@@ -358,6 +361,7 @@ enum {
  * 'wq_power_efficient' is disabled.  See WQ_POWER_EFFICIENT for more info.
  */
 extern struct workqueue_struct *system_wq;
+extern struct workqueue_struct *system_highpri_wq;
 extern struct workqueue_struct *system_long_wq;
 extern struct workqueue_struct *system_unbound_wq;
 extern struct workqueue_struct *system_freezable_wq;