Change PGT to a sealed class

This one is a bit more necessary as we have a virtual call inside PGT,
and if it isn't a sealed class, we may end up at a null dereference
under a race condition (which theoretically still *shouldn't* happen,
but rather be safe than sorry). This also brings it inline with PCD.

PKHeX/MysteryGifts/PGT.cs

@@ -100,7 +100,7 @@ public override PKM convertToPKM(SaveFile SAV)
             return Gift.convertToPKM(SAV);
         }
     }
-    public class PGT : MysteryGift
+    public sealed class PGT : MysteryGift
     {
         internal const int Size = 0x104; // 260
         public override int Format => 4;