Implementing face clean up callback in forwarding strategy

Necessary for FwStats strategy that uses CcnxFace object as a key to
several statistics parameters.
diff --git a/utils/load-stats-node.h b/utils/load-stats-node.h
index 9bdf5be..a4d6d87 100644
--- a/utils/load-stats-node.h
+++ b/utils/load-stats-node.h
@@ -118,6 +118,9 @@
     // don't do any copying at all
     return *this;
   }
+
+  void
+  RemoveFace (ns3::Ptr<ns3::CcnxFace> face);
   
 private:
   LoadStatsFace   m_pit;