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>

test/run_filter.cxx

@@ -42,8 +42,8 @@
 #include <stdio.h>
 
 void
-mixer_set_volume(gcc_unused Mixer *mixer,
-		 gcc_unused unsigned volume)
+mixer_set_volume([[maybe_unused]] Mixer *mixer,
+		 [[maybe_unused]] unsigned volume)
 {
 }