Move s_using_custom_client to DiscordPresence.cpp

Otherwise, files that include the header get warning: ‘Discord::s_using_custom_client’ defined but not used.

Source/Core/UICommon/DiscordPresence.cpp

@@ -24,6 +24,8 @@
 
 namespace Discord
 {
+static bool s_using_custom_client = false;
+
 #ifdef USE_DISCORD_PRESENCE
 namespace
 {

Source/Core/UICommon/DiscordPresence.h

@@ -27,8 +27,6 @@ enum class SecretType
   RoomID,
 };
 
-static bool s_using_custom_client = false;
-
 void Init();
 void InitNetPlayFunctionality(Handler& handler);
 void CallPendingCallbacks();