js: Explicitly set expand of St.Bin children

Right now, St.Bin ignores the child's expand properties, and
instead expands the child if its alignment is set to FILL.

This will change when we replace the custom allocation code
with a layout manager, so to prepare for that, set the expand
property where needed to keep the expected behavior.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3046>

js/gdm/loginDialog.js

@@ -50,6 +50,7 @@ export const UserListItem = GObject.registerClass({
     _init(user) {
         let layout = new St.BoxLayout({
             vertical: true,
+            x_expand: true,
         });
         super._init({
             style_class: 'login-dialog-user-list-item',