fixed the override warning

```
In file included from citra/src/audio_core/sink_details.cpp:11:
citra/src/./audio_core/sdl2_sink.h:25:10: warning: 'SetDevice' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
    void SetDevice(int device_id);
         ^
citra/src/./audio_core/sink.h:39:18: note: overridden virtual function is here
    virtual void SetDevice(int device_id) = 0;
                 ^
```