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/output/plugins/httpd/HttpdOutputPlugin.cxx

@@ -121,7 +121,7 @@ HttpdOutput::OnDeferredBroadcast() noexcept
 
 void
 HttpdOutput::OnAccept(UniqueSocketDescriptor fd,
-		      SocketAddress, gcc_unused int uid) noexcept
+		      SocketAddress, [[maybe_unused]] int uid) noexcept
 {
 	/* the listener socket has become readable - a client has
 	   connected */