citra_qt/main: Apply the [[maybe_unused]] attribute to the parameter of SetDiscordEnabled()

Depending on whether or not USE_DISCORD_PRESENCE is defined, the "state"
parameter can be used or unused. If USE_DISCORD_PRESENCE is not defined,
the parameter will be considered unused, which can lead to compiler
warnings. So, we can explicitly mark it with [[maybe_unused]] to inform
the compiler that this is intentional.

src/citra_qt/main.cpp

@@ -1714,7 +1714,7 @@ void GMainWindow::RetranslateStatusBar() {
     multiplayer_state->retranslateUi();
 }
 
-void GMainWindow::SetDiscordEnabled(bool state) {
+void GMainWindow::SetDiscordEnabled([[maybe_unused]] bool state) {
 #ifdef USE_DISCORD_PRESENCE
     if (state) {
         discord_rpc = std::make_unique<DiscordRPC::DiscordImpl>();