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/WavpackDecoderPlugin.cxx

@@ -143,7 +143,7 @@ format_samples_int(void *buffer, uint32_t count)
  * No conversion necessary.
  */
 static void
-format_samples_nop(gcc_unused void *buffer, gcc_unused uint32_t count)
+format_samples_nop([[maybe_unused]] void *buffer, [[maybe_unused]] uint32_t count)
 {
 	/* do nothing */
 }