examples: Fix running wifi-spatial-reuse example with OBSS PD disabled

examples/wireless/wifi-spatial-reuse.cc

@@ -345,6 +345,8 @@ main(int argc, char* argv[])
     Config::Connect("/NodeList/*/ApplicationList/*/$ns3::PacketSocketServer/Rx",
                     MakeCallback(&SocketRx));
 
+    if (enableObssPd)
+    {
         // Obtain pointers to the ObssPdAlgorithm objects and hook trace sinks
         // to the Reset trace source on each STA.  Note that this trace connection
         // cannot be done through the Config path system, so pointers are used.
@@ -356,6 +358,7 @@ main(int argc, char* argv[])
         auto hePhyB = DynamicCast<HePhy>(deviceB->GetPhy()->GetPhyEntity(WIFI_MOD_CLASS_HE));
         // Pass in the context string "2" to allow the trace to distinguish objects
         hePhyB->GetObssPdAlgorithm()->TraceConnect("Reset", "2", MakeCallback(&ResetTrace));
+    }
 
     Simulator::Stop(duration);
     Simulator::Run();