qt: adds default value for qt.platform (#884)

Add a default value for the qt.platform option, following commit d171b19c1c
("qt: add temporary warnings for plasma6 (#845)") in an effort to resolve [1]
("Stylix Kvantum is breaking plasma6").

[1]: https://github.com/danth/stylix/issues/835

Link: https://github.com/danth/stylix/pull/884

Reviewed-by: NAHO <90870942+trueNAHO@users.noreply.github.com>

modules/qt/nixos.nix

@@ -24,6 +24,7 @@ in
         Defaults to the standard platform used in the configured DE.
       '';
       type = lib.types.str;
+      default = "qtct";
     };
   };