workqueue: Add a workqueue chapter to the tracepoint docbook

Add a workqueue chapter to the traceopint docbook.

Current book is at: http://www.kernel.org/doc/htmldocs/tracepoint/Signed-off-by: NJason Baron <jbaron@redhat.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

Documentation/DocBook/tracepoint.tmpl

@@ -104,4 +104,9 @@
    <title>Block IO</title>
 !Iinclude/trace/events/block.h
   </chapter>
+
+  <chapter id="workqueue">
+   <title>Workqueue</title>
+!Iinclude/trace/events/workqueue.h
+  </chapter>
 </book>