Added set_platform_accelerator() for nvd_device.h for OpenCL compatibility

  /// List all devices along with all properties

  inline void print_all(std::ostream &out);

  /// Select the platform that has accelerators (for compatibility with OpenCL)

  inline int set_platform_accelerator(int pid=-1) { return UCL_SUCCESS; }

 private:

  int _device, _num_devices;

  std::vector<NVDProperties> _properties;