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/lib/xiph/FlacIOHandle.cxx

@@ -119,7 +119,7 @@ FlacIOEof(FLAC__IOHandle handle)
 }
 
 static int
-FlacIOClose(gcc_unused FLAC__IOHandle handle)
+FlacIOClose([[maybe_unused]] FLAC__IOHandle handle)
 {
 	/* no-op because the libFLAC caller is responsible for closing
 	   the #InputStream */