message: Fix build break on Mac.
  CL overlooked the use of mozc::once_t in generate_mapping.py. This CL
  replaces mozc::once_t by absl::once_flag for generated sources.
diff: @@ -73,7 +73,7 @@ class Mapping(object):
  namespace {
  static std::map<%(key_type)s, %(result_type)s> *k%(mapname)s = nullptr;
  static std::map<%(key_type)s, %(result_type)s> *k%(mapname)sShift = nullptr;
  -static once_t kOnceFor%(mapname)s = MOZC_ONCE_INIT;
  +static absl::once_flag kOnceFor%(mapname)s;
  void Init%(mapname)s() {
  if (k%(mapname)s != nullptr || k%(mapname)sShift != nullptr) {
  return;
  