Fix PlayStation build following 278653@main

https://bugs.webkit.org/show_bug.cgi?id=274065

Unreviewed build fix.

* Source/WebCore/accessibility/playstation/AXObjectCachePlayStation.cpp:
(WebCore::AXObjectCache::postPlatformNotification):
Explicitly cast AXCoreObject to AccessibilityObject.

Canonical link: https://commits.webkit.org/278669@main

Source/WebCore/accessibility/playstation/AXObjectCachePlayStation.cpp

@@ -88,7 +88,7 @@ void AXObjectCache::postPlatformNotification(AccessibilityObject& object, AXNoti
     RefPtr protectedObject = &object;
     switch (notification) {
     case AXNotification::AXSelectedChildrenChanged:
-        protectedObject = notifyChildrenSelectionChange(protectedObject.get());
+        protectedObject = dynamicDowncast<AccessibilityObject>(notifyChildrenSelectionChange(protectedObject.get()));
         break;
     case AXNotification::AXPressDidSucceed:
         notification = checkInteractableObjects(protectedObject.get());