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/encoder/plugins/WaveEncoderPlugin.cxx

@@ -98,7 +98,7 @@ fill_wave_header(WaveHeader *header, int channels, int bits,
 }
 
 static PreparedEncoder *
-wave_encoder_init(gcc_unused const ConfigBlock &block)
+wave_encoder_init([[maybe_unused]] const ConfigBlock &block)
 {
 	return new PreparedWaveEncoder();
 }