message: Add missing `override` annotation to make compiler happy
  Removes build-time compiler warnings due to overriding a member function
  without an explicit `override` marker due to `-Winconsistent-missing-override`
  directive.
diff: @@ -34,7 +34,7 @@ class CollectiveRemoteAccessLocal : public PerStepCollectiveRemoteAccess {
  virtual ~CollectiveRemoteAccessLocal() {}
  - void StartAbort(const Status& s);
  + void StartAbort(const Status& s) override;
  void RecvFromPeer(const string& peer_device, const string& peer_task,
  bool peer_is_local, const string& key, Device* to_device,
  