Bump MODULE_VERSION to 5.2.1

Change-Id: Ie52035bd9eec3fffb2c2b2b6b9869dd23f4ace3f
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Matti Paaso <matti.paaso@digia.com>