AchievementManager: Add required forward declarations

This was causing compilation errors when building without
USE_RETRO_ACHIEVEMENTS.

Source/Core/Core/AchievementManager.h

@@ -275,11 +275,21 @@ private:
 
 #include <string>
 
+namespace ActionReplay
+{
+struct ARCode;
+}
+
 namespace DiscIO
 {
 class Volume;
 }
 
+namespace Gecko
+{
+class GeckoCode;
+}
+
 class AchievementManager
 {
 public: