Add a weak reference to Webkit framework for iOS
Add a weak reference to Webkit framework for iOS

The `WKWebView` class is referenced in the code [here](https://github.com/Telerik-Verified-Plugins/Geofencing/blob/master/src/ios/GeofencePlugin.swift#L152) which requires a weak link to the Webkit framework. Without said weak link Xcode fails the build during the linking phase with the error: 
```
Undefined symbols for architecture arm64:
  "_OBJC_CLASS_$_WKWebView", referenced from:
      function signature specialization <Arg[0] = Owned To Guaranteed and Exploded> of Cordova500.GeofencePlugin.evaluateJs (Swift.String) -> () in GeofencePlugin.o
```
Telerik-Verified-Plugins/Geofencing,Telerik-Verified-Plugins/Geofencing,Telerik-Verified-Plugins/Geofencing,Telerik-Verified-Plugins/Geofencing