Qt: Hook up audio driver selection

src/core/settings.h

@@ -144,6 +144,7 @@ struct Settings
 
   AudioBackend audio_backend = DEFAULT_AUDIO_BACKEND;
   AudioStretchMode audio_stretch_mode = DEFAULT_AUDIO_STRETCH_MODE;
+  std::string audio_driver;
   u32 audio_output_latency_ms = DEFAULT_AUDIO_OUTPUT_LATENCY_MS;
   u32 audio_buffer_ms = DEFAULT_AUDIO_BUFFER_MS;
   u32 audio_output_volume = 100;