udiskie: fix getExe warning

Adding a package option to udiskie created an evaluation warning as
there is no default executable for udiskie. This replaces `pkgs.getExe`
with `pkgs.getExe'`, which removes this evaluation warning, but does
require that the package has a binary named udiskie.

modules/services/udiskie.nix

@@ -135,7 +135,7 @@ in
       };
 
       Service.ExecStart = toString (
-        [ (lib.getExe cfg.package) ]
+        [ (lib.getExe' cfg.package "udiskie") ]
         ++ lib.optional config.xsession.preferStatusNotifierItems "--appindicator"
       );