dialogs: let QtQuick.Dialogs depend on QtQuick.Dialogs.Private

With need to add an explicit dependency on QtQuick.Dialogs.Private
in qmldir, since plugin.cpp decides run-time if it needs to load
private qml files.

Change-Id: I324858586a6c0b67c00eb7f8160c812a81caa21d
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>