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/[e-mail]

Reviewed-by: Todd Poynor <[e-mail]>
Signed-off-by: Ahbong Chang <[e-mail]>
Signed-off-by: Steven Rostedt (VMware) <[e-mail]>

include/trace/events/workqueue.h

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