static builds: add classname entry to qmldir for QtQuick.Dialogs.Private

Add classname entry to qmldir, otherwise qmlimportscanner cannot
utilize it when doing static builds (on iOS).

Task-number: QTBUG-35369

Change-Id: If5440ffcdd805a832d4ff1d129e9f24d222b45d5
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>