network: Fix mtu label not requesting enough space
Previously, changing the MTU value to non-automatic shows the "byte" label. But it overlays with the spin button.
Fix that by calling gtk_widget_queue_resize() before showing/hiding that label.
This[1] is the related issue in gtk+. Ideally, this should be done while calling gtk_widget_show/hide().
*[1] gtk#1560 (closed)