helper: Fixing bug with ndn::GlobalRoutingHelper

With certain metric parameters, the value was incorrectly wrapped
around zero, leading to incorrect route calculation (BoostGrapLibrary
was detecting negative edge weights).
diff --git a/helper/boost-graph-ndn-global-routing-helper.h b/helper/boost-graph-ndn-global-routing-helper.h
index a223a82..a85e9c9 100644
--- a/helper/boost-graph-ndn-global-routing-helper.h
+++ b/helper/boost-graph-ndn-global-routing-helper.h
@@ -221,8 +221,8 @@
                                 bool>
 {
   bool
-  operator () (property_traits< EdgeWeights >::reference a,
-               property_traits< EdgeWeights >::reference b) const
+  operator () (tuple< ns3::Ptr<ns3::ndn::Face>, uint32_t, double > a,
+               tuple< ns3::Ptr<ns3::ndn::Face>, uint32_t, double > b) const
   {
     return a.get<1> () < b.get<1> ();
   }