programs/calls: enable dconf

GNOME Calls relies on dconf. Evidence supporting this claim:
https://packages.debian.org/sid/gnome-calls depends on
dconf-gsettings-backend.

Discussed in
https://github.com/NixOS/nixpkgs/pull/136537#issuecomment-1009903194 :

nixos/modules/programs/calls.nix

···
   };
 
   config = mkIf cfg.enable {
+    programs.dconf.enable = true;
+
     environment.systemPackages = [
       pkgs.calls
     ];