Fix build with msvc2010

override on destructors are not very useful anyhow, but also msvc2010
doesn't like them at all.

Change-Id: I2d5f4b0d6148c5e66d2cba604d305707269d12e0
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>