remove gcc_unused

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/mixer/plugins/OSXMixerPlugin.cxx

@@ -54,9 +54,9 @@ OSXMixer::SetVolume(unsigned new_volume)
 }
 
 static Mixer *
-osx_mixer_init(gcc_unused EventLoop &event_loop, AudioOutput &ao,
+osx_mixer_init([[maybe_unused]] EventLoop &event_loop, AudioOutput &ao,
 		MixerListener &listener,
-		gcc_unused const ConfigBlock &block)
+		[[maybe_unused]] const ConfigBlock &block)
 {
 	OSXOutput &osxo = (OSXOutput &)ao;
 	return new OSXMixer(osxo, listener);