utils: Add tracing functionality of network-layer NACKs

Change-Id: I3d16bcf29f4858049d1040a3e421e1c7151a1ea1
Refs: #3872
diff --git a/utils/tracers/ndn-l3-tracer.cpp b/utils/tracers/ndn-l3-tracer.cpp
index e19439b..64c93ce 100644
--- a/utils/tracers/ndn-l3-tracer.cpp
+++ b/utils/tracers/ndn-l3-tracer.cpp
@@ -61,6 +61,8 @@
   l3->TraceConnectWithoutContext("InInterests", MakeCallback(&L3Tracer::InInterests, this));
   l3->TraceConnectWithoutContext("OutData", MakeCallback(&L3Tracer::OutData, this));
   l3->TraceConnectWithoutContext("InData", MakeCallback(&L3Tracer::InData, this));
+  l3->TraceConnectWithoutContext("OutNack", MakeCallback(&L3Tracer::OutNack, this));
+  l3->TraceConnectWithoutContext("InNack", MakeCallback(&L3Tracer::InNack, this));
 
   // satisfied/timed out PIs
   l3->TraceConnectWithoutContext("SatisfiedInterests",