DolphinQt/Android: Add warning when converting NKit files

Yes, that's right! It's time to add even more NKit warnings,
because users still don't understand what NKit is or how it works!

More specifically, some users seem to be under the impression that
converting an NKit file to for instance RVZ using Dolphin's convert
feature will result in a normal RVZ file, when it in fact results in
an NKit RVZ file (since NKit is not a container format in the sense
that GCZ/WIA/RVZ/WBFS/CISO is, but rather a kind of trimmed ISO).
I can hardly blame users for not knowing this, because it's not
intuitive unless you know the technical details of how NKit works.

Source/Core/UICommon/GameFile.h

@@ -106,6 +106,7 @@ public:
   u64 GetVolumeSize() const { return m_volume_size; }
   bool IsVolumeSizeAccurate() const { return m_volume_size_is_accurate; }
   bool IsDatelDisc() const { return m_is_datel_disc; }
+  bool IsNKit() const { return m_is_nkit; }
   const GameBanner& GetBannerImage() const;
   const GameCover& GetCoverImage() const;
   void DoState(PointerWrap& p);