Fix memory leak in SampleGrabberCallback

Add virtual destructor to SampleGrabberCallback to let it properly
destruct its subclass when calling 'detete this'

Change-Id: I5f74f82314e22225a0a153c7f4bca531a79dd8f9
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>