sim: Call regStats of base-class as well

We want to extend the stats of objects hierarchically and thus it is necessary
to register the statistics of the base-class(es), as well.  For now, these are
empty, but generic stats will be added there.

Patch originally provided by Akash Bagdia at ARM Ltd.

src/cpu/testers/traffic_gen/traffic_gen.cc

@@ -526,6 +526,8 @@ TrafficGen::noProgress()
 void
 TrafficGen::regStats()
 {
+    ClockedObject::regStats();
+
     // Initialise all the stats
     using namespace Stats;