Enable limits options in CcnxStackHelper

Slightly modifying the way of how forwarding strategy is hooked up
diff --git a/model/ccnx-bestroute-strategy.cc b/model/ccnx-bestroute-strategy.cc
index 20d1a81..ab525b5 100644
--- a/model/ccnx-bestroute-strategy.cc
+++ b/model/ccnx-bestroute-strategy.cc
@@ -38,9 +38,10 @@
 TypeId CcnxBestRouteStrategy::GetTypeId (void)
 {
   static TypeId tid = TypeId ("ns3::CcnxBestRouteStrategy")
-  .SetGroupName ("Ccnx")
-  .SetParent<CcnxForwardingStrategy> ()
-  ;
+    .SetGroupName ("Ccnx")
+    .SetParent <CcnxForwardingStrategy> ()
+    .AddConstructor <CcnxBestRouteStrategy> ()
+    ;
   return tid;
 }