limits: Introducing modularity for Interest limits

Now ndn::Limits object can be aggregated on a face/FIB entry. A
forwarding strategy module is responsible in creating an appropriate
implementation of the Interest limits module.
diff --git a/model/fw/ndn-forwarding-strategy.cc b/model/fw/ndn-forwarding-strategy.cc
index 9fc4d29..b04d48e 100644
--- a/model/fw/ndn-forwarding-strategy.cc
+++ b/model/fw/ndn-forwarding-strategy.cc
@@ -528,6 +528,11 @@
   // do nothing for now. may be need to do some logging
 }
 
+void
+ForwardingStrategy::AddFace (Ptr<Face> face)
+{
+  // do nothing here
+}
 
 void
 ForwardingStrategy::RemoveFace (Ptr<Face> face)