Fix protobuf status error

[veblush]

Fixed prod:grpc/core/master/linux/grpc_build_protobuf_at_head

/var/local/git/grpc/test/cpp/qps/parse_json.cc: In function 'void grpc::testing::ParseJson(const string&, const string&, google::protobuf::Message*)':
/var/local/git/grpc/test/cpp/qps/parse_json.cc:37:31: error: 'class google::protobuf::util::status_internal::Status' has no member named 'error_message'
     std::string errmsg(status.error_message());

Related with protocolbuffers/protobuf#8354