networkmanager: enable polkit_agent

Not sure why we had this disabled.
This is for the nmcli/nmtui `agent` subcommand [0]
which allows it to act as a polkit agent.

> nmcli agent { secret | polkit | all }
> Run nmcli as a NetworkManager secret agent, or polkit agent.

For example, this is useful if you want to use nmcli/nmtui to authenti-
cate with polkit on a headless server.

[0]: https://developer.gnome.org/NetworkManager/stable/nmcli.html

pkgs/tools/networking/network-manager/default.nix

@@ -41,6 +41,7 @@ in stdenv.mkDerivation rec {
     "-Dcrypto=gnutls"
     "-Dsession_tracking=systemd"
     "-Dmodem_manager=true"
+    "-Dpolkit_agent=true"
     "-Dnmtui=true"
     "-Ddocs=true"
     "-Dtests=no"