Enabling app-layer tracing

Network-layer tracing is not working yet (needs a couple of attributes
to be implemented)
diff --git a/apps/ccnx-producer.h b/apps/ccnx-producer.h
index ac9a492..a239d84 100644
--- a/apps/ccnx-producer.h
+++ b/apps/ccnx-producer.h
@@ -52,6 +52,9 @@
 private:
   CcnxNameComponents m_prefix;
   uint32_t m_virtualPayloadSize;
+
+  TracedCallback<Ptr<const CcnxContentObjectHeader>, Ptr<const Packet>,
+                 Ptr<CcnxApp>, Ptr<CcnxFace> > m_transmittedContentObjects;
 };
 
 }