Add license for XPathQuery files

[lundstroem]

This is what's used currently:

//
//  XPathQuery.h
//  FuelFinder
//
//  Created by Matt Gallagher on 4/08/08.
//  Copyright 2008 __MyCompanyName__. All rights reserved.
//

Original article: https://www.cocoawithlove.com/2008/10/using-libxml2-for-parsing-and-xpath.html

Here's the license specified: https://www.cocoawithlove.com/about/

// Copyright © 2008-2018 Matt Gallagher ( http://cocoawithlove.com ). All rights reserved.

// Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee
// is hereby granted, provided that the above copyright notice and this permission notice appear in
// all copies.

// THE SOFTWARE IS PROVIDED “AS IS” AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING
// FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
// NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH
// THE USE OR PERFORMANCE OF THIS SOFTWARE.