Added an XPC Service template

Includes a basic template for creating an XPC service that runs via a launch daemon (similar to theos' XPC template). Currently does not include sample code for an XPC client that uses the service, but writing such a client is fairly trivial (and there's plenty of documentation on it).

This template relies on access to XPC framework headers, which I've added in my iOSOpenDev fork (in the include directory) and can submit a pull request for too.