sessionMode: Enable networkAgent on login screen

We will soon enable the network sections in the status menu on the
login screen, so enable the network agent to handle authentication
requests (like wifi/VPN passwords).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1874>

js/ui/sessionMode.js

@@ -47,7 +47,9 @@ const _modes = {
         isGreeter: true,
         isPrimary: true,
         unlockDialog: imports.gdm.loginDialog.LoginDialog,
-        components: ['polkitAgent'],
+        components: Config.HAVE_NETWORKMANAGER
+            ? ['networkAgent', 'polkitAgent']
+            : ['polkitAgent'],
         panel: {
             left: [],
             center: ['dateMenu'],