message: Fix warning when building with -Wthread-safety-precise in function.cc.
diff: @@ -1226,6 +1226,7 @@ Status FunctionLibraryDefinition::AddLibrary(
  // the duration of the function could lead to deadlock).
  FunctionLibraryDefinition clone(other);
  mutex_lock l(mu_);
  + mutex_lock l2(clone.mu_);
  // Remember the funcs and grads that we added successfully so that
  // we can roll them back on error.
  std::vector<string> funcs;
  