traced_perf: Include kernel return addresses in stack traces

These are sometimes useful for identifying the call site (e.g. in case
of an indirect call or multiple calls to the same function).

Author: pcc

src/profiling/perf/unwinding.cc

@@ -526,6 +526,7 @@ std::vector<unwindstack::FrameData> Unwinder::SymbolizeKernelCallchain(
     unwindstack::FrameData& frame = ret.emplace_back();
     frame.function_name = kernel_map->Lookup(ip);
     frame.map_info = kernel_map_info.ref();
+    frame.rel_pc = ip;
   }
   return ret;
 }