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/model/forwarding-strategy/fw-stats.h b/model/forwarding-strategy/fw-stats.h
index 1849fb2..67f2763 100644
--- a/model/forwarding-strategy/fw-stats.h
+++ b/model/forwarding-strategy/fw-stats.h
@@ -56,7 +56,10 @@
           Ptr<CcnxContentObjectHeader> &header,
           Ptr<Packet> &payload,
           const Ptr<const Packet> &packet);
-  
+
+  virtual void
+  RemoveFace (Ptr<CcnxFace> face);
+
 protected:
   virtual void
   DidCreatePitEntry (const Ptr<CcnxFace> &incomingFace,