yuzu/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.