unlockDialog: Fix centering of label in screen shield

The code here was trying to center the label, but that didn't
happen because we allocated the entire space to the label, which
still plonks it at the top.

js/ui/unlockDialog.js

@@ -41,6 +41,7 @@ const UserWidget = new Lang.Class({
         this.actor.add(this._label,
                        { expand: true,
                          x_fill: true,
+                         y_fill: false,
                          y_align: St.Align.MIDDLE });
 
         this._userLoadedId = this._user.connect('notify::is-loaded',