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>