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/decoder/plugins/VorbisDecoderPlugin.cxx

@@ -311,7 +311,7 @@ VorbisDecoder::OnOggEnd()
 /* public */
 
 static bool
-vorbis_init(gcc_unused const ConfigBlock &block)
+vorbis_init([[maybe_unused]] const ConfigBlock &block)
 {
 #ifndef HAVE_TREMOR
 	LogDebug(vorbis_domain, vorbis_version_string());