message: [style] fix a warning on c++11
diff: @@ -49,12 +49,16 @@ struct TestStruct
  std::string name;
  };
  -template <> struct otbr::DBus::DBusTypeTrait<TestStruct>
  +namespace otbr {
  +namespace DBus {
  +template <> struct DBusTypeTrait<TestStruct>
  {
  static constexpr const char *TYPE_AS_STRING =
  //{uint8, uint32, string}
  "(yus)";
  };
  +} // namespace DBus
  +} // namespace otbr
  bool operator==(const TestStruct &aLhs, const TestStruct &aRhs)
  {
  