Enable limits options in CcnxStackHelper

Slightly modifying the way of how forwarding strategy is hooked up
diff --git a/model/ccnx-flooding-strategy.cc b/model/ccnx-flooding-strategy.cc
index 147edba..ee7df67 100644
--- a/model/ccnx-flooding-strategy.cc
+++ b/model/ccnx-flooding-strategy.cc
@@ -40,7 +40,8 @@
 {
   static TypeId tid = TypeId ("ns3::CcnxFloodingStrategy")
     .SetGroupName ("Ccnx")
-    .SetParent<CcnxForwardingStrategy> ()
+    .SetParent <CcnxForwardingStrategy> ()
+    .AddConstructor <CcnxFloodingStrategy> ()
     ;
   return tid;
 }