Building for iOS simulator yields linker warnings and errors

```
ld: warning: building for iOS Simulator, but linking in .tbd file (/opt/platformsdk/iPhoneSimulator14.0.sdk//usr/lib/system/libsystem_kernel.tbd) built for macOS
ld: warning: building for iOS Simulator, but linking in .tbd file (/opt/platformsdk/iPhoneSimulator14.0.sdk//usr/lib/system/libsystem_platform.tbd) built for macOS
ld: warning: building for iOS Simulator, but linking in .tbd file (/opt/platformsdk/iPhoneSimulator14.0.sdk//usr/lib/system/libsystem_pthread.tbd) built for macOS
Undefined symbols for architecture x86_64:
"_OBJC_CLASS_$_WKWebView", referenced from:
objc-class-ref in libWebViewExternal_1.a(webview_darwin.mm_2.o)
"_OBJC_CLASS_$_WKWebViewConfiguration", referenced from:
objc-class-ref in libWebViewExternal_1.a(webview_darwin.mm_2.o)
```

The linker error I fixed by adding the WebKit framework to the extension manifest for x86_64-ios, but the warnings are still there. Not sure if they have any impact.