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/event/ServerSocket.cxx

@@ -161,7 +161,7 @@ ServerSocket::OneServerSocket::Accept() noexcept
 }
 
 bool
-ServerSocket::OneServerSocket::OnSocketReady(gcc_unused unsigned flags) noexcept
+ServerSocket::OneServerSocket::OnSocketReady([[maybe_unused]] unsigned flags) noexcept
 {
 	Accept();
 	return true;