Fix GtkSwitchAccessible type definition

Fix GtkSwitchAccessible type definition

It turns out that ATK_DEFINE_TYPE_WITH_CODE() is broken; it
tells GType that the class and instance size for the accessible
type are the same as for its parent type. Which is not true
if your instance struct has members such as 'description' here.
This was causing hard-to-track-down memory corruption, since
description and the GtkAccessible private pointer were sharing
the same memory location.