tracing: Add missing forward declaration

Without this forward declaration compile may fail if this header is
included only for registering other probe event without struct
pool_workqueue.

Link: http://lkml.kernel.org/r/20180416023626.139915-1-cwahbong@google.com

Reviewed-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Ahbong Chang <cwahbong@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/trace/events/workqueue.h

@@ -25,6 +25,8 @@ DECLARE_EVENT_CLASS(workqueue_work,
 	TP_printk("work struct %p", __entry->work)
 );
 
+struct pool_workqueue;
+
 /**
  * workqueue_queue_work - called when a work gets queued
  * @req_cpu:	the requested cpu