nixos-generate-config: add 'lib' to module template

The NixOS manual says modules have the following signature:
 { config, lib, pkgs, ... }:

But our generated configuration.nix file lacks the 'lib' part. Add it.
Merge pull request #5016 from bjornfor/nixos-generate-config-with-lib

nixos-generate-config: add 'lib' to module template