Fix warning

MainConnection inherits ErrorLogger, which contains a virtual function

/usr/include/qt4/QtCore/qsharedpointer_impl.h:342:21: warning: deleting object
of polymorphic class type ‘ZeroConf::Internal::MainConnection’ which has
non-virtual destructor might cause undefined behaviour [-Wdelete-non-virtual-dtor]

Change-Id: I6e0a698da8f1b7024eae24167078b51e8921a435
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: hjk <qthjk@ovi.com>