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/OpusDecoderPlugin.cxx

@@ -64,7 +64,7 @@ IsOpusTags(const ogg_packet &packet) noexcept
 }
 
 bool
-mpd_opus_init(gcc_unused const ConfigBlock &block)
+mpd_opus_init([[maybe_unused]] const ConfigBlock &block)
 {
 	LogDebug(opus_domain, opus_get_version_string());