vassal: correct platforms

Upstream also supports Windows

pkgs/by-name/va/vassal/package.nix

···
     sourceProvenance = with sourceTypes; [ binaryBytecode ];
     license = licenses.lgpl21Only;
     maintainers = with maintainers; [ tvestelind ];
-    platforms = platforms.unix;
+    platforms = with lib.platforms; unix ++ windows;
     mainProgram = "vassal";
   };
 }