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/stats-tree.h b/utils/stats-tree.h
index 9702675..1bb128b 100644
--- a/utils/stats-tree.h
+++ b/utils/stats-tree.h
@@ -65,10 +65,14 @@
   // Get (const ns3::CcnxNameComponents &key) const;
   const LoadStatsNode &
   operator [] (const ns3::CcnxNameComponents &key) const;
+
+  void
+  RemoveFace (ns3::Ptr<ns3::CcnxFace> face);
   
 private:
   const LoadStatsNode &
   WalkLeftRightRoot (tree_type *node);
+
   
 private:
   tree_type m_tree;