cpu: Fix InstPBTrace inheritance

This patch fixes an issue that prevented gem5 to be built with C++
config and without Python.

src/cpu/inst_pb_trace.hh

@@ -82,7 +82,7 @@ class InstPBTraceRecord : public InstRecord
 
 };
 
-class InstPBTrace : InstTracer
+class InstPBTrace : public InstTracer
 {
  public:
     InstPBTrace(const InstPBTraceParams *p);