Define a module map for libxml2 #69

If I use use_frameworks! with Cocoapods and include the RaptureXML pod, it fails to compile. See CocoaPods/CocoaPods#3654 (comment)

This can be recreated by setting up a new single view project and adding the RaptureXML pod with use_frameworks! set in the pod.

On that linked issue, while we wait for Apple to provide the module map, they suggest defining your own for libxml in your Cocoapod like this:
https://github.com/neonichu/SQLite.swift/blob/master/SQLite.swift.podspec#L25-L33