Mark the missing QDnsLookup constructor as \internal

Implementation will be added for Qt 5.4.0 as it cannot be done
sooner.

Change-Id: I4d2626416fae99339988cd994653ce7ec753f081
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>