Try to fix older visionOS builds (linker)

https://bugs.webkit.org/show_bug.cgi?id=270763
<rdar://124349498>

Reviewed by Tim Horton.

`WKSPreviewWindowController.h` needs to be part of `WebKitSwift.h`
because of the delegate. So it can't be included only based on a
(compile-time) feature flag.
Introduce a stub `WKSPreviewWindowController` instead for older SDKs,
where the feature is turned off anyway.

* Source/WebKit/WebKitSwift/Preview/PreviewWindowController.swift:

Canonical link: https://commits.webkit.org/275893@main

Source/WebKit/WebKitSwift/Preview/PreviewWindowController.swift

@@ -81,6 +81,10 @@ public final class PreviewWindowController: NSObject {
     }
 }
 
+#else
+import Foundation
+@objc(WKSPreviewWindowController)
+public final class PreviewWindowController: NSObject { }
 #endif
 
 #endif