Avoid initializing ASC softlink when in base system

https://bugs.webkit.org/show_bug.cgi?id=258888
rdar://111698557

Reviewed by Alexey Proskuryakov.

Use SOFT_LINK_CLASS_FOR_SOURCE_OPTIONAL to avoid crashing when ASC isn't present
on base system.

* Source/WebKit/UIProcess/WebAuthentication/Cocoa/LocalService.mm:
(WebKit::LocalService::isAvailable):
* Source/WebKit/UIProcess/WebAuthentication/Cocoa/WebAuthenticatorCoordinatorProxy.mm:

Canonical link: https://commits.webkit.org/265870@main

Source/WebKit/UIProcess/WebAuthentication/Cocoa/AuthenticationServicesCoreSoftLink.mm

@@ -30,7 +30,7 @@
 
 SOFT_LINK_PRIVATE_FRAMEWORK_FOR_SOURCE(WebKit, AuthenticationServicesCore);
 
-SOFT_LINK_CLASS_FOR_SOURCE(WebKit, AuthenticationServicesCore, ASCWebKitSPISupport);
+SOFT_LINK_CLASS_FOR_SOURCE_OPTIONAL(WebKit, AuthenticationServicesCore, ASCWebKitSPISupport);
 
 #if HAVE(ASC_AUTH_UI) || HAVE(UNIFIED_ASC_AUTH_UI)