Fix an error in pure virtual function in earlier CL

 public:

  virtual ~HearingAidAudioReceiver() = default;

  virtual void OnAudioDataReady(const std::vector<uint8_t>& data) = 0;

  virtual void OnAudioSuspend(std::promise<void> do_suspend_promise);

  virtual void OnAudioResume(std::promise<void> do_resume_promise);

  virtual void OnAudioSuspend(std::promise<void> do_suspend_promise) = 0;

  virtual void OnAudioResume(std::promise<void> do_resume_promise) = 0;

};

class HearingAid {