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/ffmpeg/LogCallback.cxx

@@ -47,7 +47,7 @@ FfmpegImportLogLevel(int level) noexcept
 }
 
 void
-FfmpegLogCallback(gcc_unused void *ptr, int level, const char *fmt, std::va_list vl)
+FfmpegLogCallback(void *ptr, int level, const char *fmt, std::va_list vl)
 {
 	const AVClass * cls = nullptr;