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/db/update/InotifySource.cxx

@@ -31,7 +31,7 @@
 #include <sys/inotify.h>
 
 bool
-InotifySource::OnSocketReady(gcc_unused unsigned flags) noexcept
+InotifySource::OnSocketReady([[maybe_unused]] unsigned flags) noexcept
 {
 	uint8_t buffer[4096];
 	static_assert(sizeof(buffer) >= sizeof(struct inotify_event) + NAME_MAX + 1,