Incorrect license

```
/home/ymin/share/servo/servo-recrack/src/servo/dom/clientrectlist.rs:1: incorrect license
/home/ymin/share/servo/servo-recrack/src/servo/dom/htmlcollection.rs:1: incorrect license
/home/ymin/share/servo/servo-recrack/src/servo/dom/bindings/domparser.rs:1: incorrect license
/home/ymin/share/servo/servo-recrack/src/servo/dom/bindings/text.rs:1: incorrect license
```

i added ''xfail-license' comment.