Show NKitness in file format string

To make people more aware that they're not using a normal disc image.

Source/Core/UICommon/GameFile.h

@@ -145,6 +145,7 @@ private:
   u64 m_volume_size{};
   bool m_volume_size_is_accurate{};
   bool m_is_datel_disc{};
+  bool m_is_nkit{};
 
   std::map<DiscIO::Language, std::string> m_short_names;
   std::map<DiscIO::Language, std::string> m_long_names;