Correct QSpiAccessibleBridge::setRootObject definition

Correct QSpiAccessibleBridge::setRootObject definition

QSpiAccessibleBridge::setRootObject was defined to require a
QAccessibleInterface* as parameter instead of QObject*, as a result of
that, QSpiAccessibleBridge::setRootObject did not reimplement
QPlatformAccessibility::setRootObject and the adaptor was not initialized.

After this change, applications should work with screen readers.

Change-Id: Iee3d39f8e0959c1d75b1e682847d266a723c141b
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>