Correct Open Source WebKit build when using Sonoma SDK

https://bugs.webkit.org/show_bug.cgi?id=261832
<rdar://problem/115795086>

Unreviewed build fix.

The changes in 258467@main defined a needed property for NSMutableURLRequest, but did not
provide the related declaration for NSURLRequest. This causes a build failure when someone
attempts to build trunk WebKit with the Open Source Sonoma SDK.

* Source/WebCore/PAL/pal/spi/cf/CFNetworkSPI.h:

Canonical link: https://commits.webkit.org/268214@main

Source/WebCore/PAL/pal/spi/cf/CFNetworkSPI.h

@@ -256,6 +256,9 @@ typedef enum {
 @property (setter=_setPrivacyProxyFailClosed:) BOOL _privacyProxyFailClosed;
 @property (readonly) BOOL _useEnhancedPrivacyMode;
 #endif
+#if HAVE(PRIVACY_PROXY_FAIL_CLOSED_FOR_UNREACHABLE_HOSTS)
+@property (readonly) BOOL _privacyProxyFailClosedForUnreachableNonMainHosts;
+#endif
 @end
 
 @interface NSURLResponse ()