message: fix compile warning about unused get_download_gz_file()
  get_download_gz_file() is not used when HAS_REMOTE_API=0, and causes:
  warning: 'std::__cxx11::string date::get_download_gz_file(const string&)' defined but not used [-Wunused-function]
  Wrap get_download_gz_file() inside #ifdef to remove the warning.
diff: @@ -269,6 +269,7 @@ get_install()
  return ref;
  }
  +#if HAS_REMOTE_API
  static
  std::string
  get_download_gz_file(const std::string& version)
  @@ -276,6 +277,7 @@ get_download_gz_file(const std::string& version)
  auto file = get_install() + version + ".tar.gz";
  return file;
  }
  +#endif // HAS_REMOTE_API
  // These can be used to reduce the range of the database to save memory
  CONSTDATA auto min_year = date::year::min();
  