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/FluidsynthDecoderPlugin.cxx

@@ -196,7 +196,7 @@ fluidsynth_file_decode(DecoderClient &client, Path path_fs)
 
 static bool
 fluidsynth_scan_file(Path path_fs,
-		     gcc_unused TagHandler &handler) noexcept
+		     [[maybe_unused]] TagHandler &handler) noexcept
 {
 	return fluid_is_midifile(path_fs.c_str());
 }