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/model/ndn-common.hpp b/model/ndn-common.hpp
index 8b1a692..7769cef 100644
--- a/model/ndn-common.hpp
+++ b/model/ndn-common.hpp
@@ -43,7 +43,9 @@
 
 ATTRIBUTE_HELPER_HEADER(Name);
 
-namespace time = ::ndn::time;
+namespace time {
+using namespace ::ndn::time;
+}
 
 using ::ndn::Exclude;