Use structs for config callback IDs

This way you can't mix up regular config callback IDs and CPU thread
config callback IDs. (It would be rather bad if you did!)

Source/Core/AudioCommon/Mixer.h

@@ -10,6 +10,7 @@
 #include "AudioCommon/SurroundDecoder.h"
 #include "AudioCommon/WaveFile.h"
 #include "Common/CommonTypes.h"
+#include "Common/Config/Config.h"
 
 class PointerWrap;
 
@@ -120,5 +121,5 @@ private:
   int m_config_timing_variance;
   bool m_config_audio_stretch;
 
-  size_t m_config_changed_callback_id;
+  Config::ConfigChangedCallbackID m_config_changed_callback_id;
 };