coreaudio: Always return 0 in handle_voice_change

coreaudio: Always return 0 in handle_voice_change

handle_voice_change() is a CoreAudio callback function as of CoreAudio type
AudioObjectPropertyListenerProc, and for the latter MacOSX.sdk/System/
Library/Frameworks/CoreAudio.framework/Headers/AudioHardware.h
says "The return value is currently unused and should always be 0.".

Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220306123410.61063-1-akihiko.odaki@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>