Fix the visionOS debug build after 292533@main

https://bugs.webkit.org/show_bug.cgi?id=290264
rdar://147657648

Reviewed by Jonathan Bedard.

Unreviewed build fix.

* Source/WebKit/UIProcess/API/Swift/WebPage+Configuration.swift:
(Configuration.showsSystemScreenTimeBlockingView):

Canonical link: https://commits.webkit.org/292546@main

Author: rr-codes

Source/WebKit/UIProcess/API/Swift/WebPage+Configuration.swift

@@ -109,7 +109,12 @@ extension WebPage {
         /// The default value is `true`. If `true`, the system Screen Time blocking view is shown when blocked by Screen Time.
         /// If `false`, a blurred view of the web content is shown instead.
         @available(visionOS, unavailable)
-        public var showsSystemScreenTimeBlockingView: Bool = true
+        public var showsSystemScreenTimeBlockingView: Bool {
+            get { _showsSystemScreenTimeBlockingView }
+            set { _showsSystemScreenTimeBlockingView = newValue }
+        }
+
+        private var _showsSystemScreenTimeBlockingView = true
 
 #if os(iOS)
         /// The types of data detectors to apply to the webpage's content.