Fix windows build with accessibility

For now the windows build will simply use the same accessibility
implementation as the rest of Qt and we don't bother with using the native
implementation.

Change-Id: Iaad6734834a562f2ee0bd951d299310e95a5c9e1
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Reviewed-by: Michael Bruning <michael.bruning@digia.com>