QtQuick.Dialogs.Private needs classname in its qmldir

Add classname entry to qmldir, otherwise qmlimportscanner cannot
utilize it when doing static builds (on iOS).  (same as change
6dbb25f944249f7b846be8b4d77165f054c5483f in qtdeclarative)

Task-number: QTBUG-35369
Change-Id: Id4dcd3e2193cd9f76db72f3244b1bde39a0d1a63
Reviewed-by: Liang Qi <liang.qi@digia.com>