message: Forward declare condition_variable
  Necessary to enable friendship with mutex
diff: @@ -31,6 +31,8 @@ namespace tensorflow {
  enum LinkerInitialized { LINKER_INITIALIZED };
  +class condition_variable;
  +
  // Mimic std::mutex + C++17's shared_mutex, adding a LinkerInitialized
  // constructor interface. This type is as fast as mutex, but is also a shared
  // lock.
  