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/ccnx-forwarding-strategy.h b/model/forwarding-strategy/ccnx-forwarding-strategy.h
index 20ef1b5..ff0dcef 100644
--- a/model/forwarding-strategy/ccnx-forwarding-strategy.h
+++ b/model/forwarding-strategy/ccnx-forwarding-strategy.h
@@ -83,6 +83,9 @@
 
   virtual void
   WillErasePendingInterest (Ptr<CcnxPitEntry> pitEntry);
+
+  virtual void
+  RemoveFace (Ptr<CcnxFace> face);
   
 protected:
   // events