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/plugins/upnp/UpnpDatabasePlugin.cxx

@@ -145,7 +145,7 @@ private:
 
 DatabasePtr
 UpnpDatabase::Create(EventLoop &, EventLoop &io_event_loop,
-		     gcc_unused DatabaseListener &listener,
+		     [[maybe_unused]] DatabaseListener &listener,
 		     const ConfigBlock &) noexcept
 {
 	return std::make_unique<UpnpDatabase>(io_event_loop);