qqwing: add darwin support

pkgs/games/qqwing/default.nix

···
     homepage = "https://qqwing.com";
     description = "Sudoku generating and solving software";
     license = licenses.gpl2;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     maintainers = with maintainers; [ ];
   };
 }