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/Mpg123DecoderPlugin.cxx

@@ -37,7 +37,7 @@
 static constexpr Domain mpg123_domain("mpg123");
 
 static bool
-mpd_mpg123_init(gcc_unused const ConfigBlock &block)
+mpd_mpg123_init([[maybe_unused]] const ConfigBlock &block)
 {
 	mpg123_init();