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/SndioMixerPlugin.cxx

@@ -45,10 +45,10 @@ public:
 };
 
 static Mixer *
-sndio_mixer_init(gcc_unused EventLoop &event_loop,
+sndio_mixer_init([[maybe_unused]] EventLoop &event_loop,
 		AudioOutput &ao,
 		MixerListener &listener,
-		gcc_unused const ConfigBlock &block)
+		[[maybe_unused]] const ConfigBlock &block)
 {
 	return new SndioMixer((SndioOutput &)ao, listener);
 }