Only add Wireguard kernel module on Kernels that do not already have it

This makes wireguard work on older kernels (19.09 nixos), i.e.
our storage servers currently) and newer kernels (21.05, i.e.
the monitoring server).

morph/lib/vagrant-guest.nix

 # Minimal configuration that vagrant depends on
 
-{ config, pkgs, ... }:
+{ config, pkgs, lib, ... }:
 let
   # Vagrant uses an insecure shared private key by default, but we
   # don't use the authorizedKeys attribute under users because it should be
@@ -22,8 +22,10 @@ in
   # Enable the OpenSSH daemon.
   services.openssh.enable = true;
 
-  # Wireguard kernel module
-  boot.extraModulePackages = [ config.boot.kernelPackages.wireguard ];
+  # Wireguard kernel module for Kernels < 5.6
+  boot = lib.mkIf (lib.versionOlder pkgs.linuxPackages.kernel.version "5.6") {
+    extraModulePackages = [ config.boot.kernelPackages.wireguard ] ;
+  };
 
   # Enable DBus
   services.dbus.enable    = true;