audio_device: Mark GetDeviceVolume as const

This doesn't modify instance state.

src/audio_core/renderer/audio_device.cpp

@@ -45,7 +45,7 @@ void AudioDevice::SetDeviceVolumes(const f32 volume) {
     output_sink.SetDeviceVolume(volume);
 }
 
-f32 AudioDevice::GetDeviceVolume([[maybe_unused]] std::string_view name) {
+f32 AudioDevice::GetDeviceVolume([[maybe_unused]] std::string_view name) const {
     return output_sink.GetDeviceVolume();
 }
 

src/audio_core/renderer/audio_device.h

@@ -73,7 +73,7 @@ public:
      * @param name - Name of the device to check. Unused.
      * @return Volume of the device.
      */
-    f32 GetDeviceVolume(std::string_view name);
+    f32 GetDeviceVolume(std::string_view name) const;
 
 private:
     /// Backend output sink for the device