utils: Enable use of the simulation time in NFD

Conversion of ndnSIM time to system_clock::time_point, steady_clock::time_point, so that NFD use the simulation time.
diff --git a/helper/ndn-stack-helper.hpp b/helper/ndn-stack-helper.hpp
index afc43dc..b6b4266 100644
--- a/helper/ndn-stack-helper.hpp
+++ b/helper/ndn-stack-helper.hpp
@@ -303,6 +303,10 @@
   PointToPointNetDeviceCallback(Ptr<Node> node, Ptr<L3Protocol> ndn,
                                 Ptr<NetDevice> netDevice) const;
 
+public:
+  void
+  setCustomNdnCxxClocks();
+
 private:
   StackHelper(const StackHelper&);
   StackHelper&