Add phpunit.xml.dist to XML filenames

PHPUnit (a popular unit testing tool for PHP) uses `phpunit.xml` for its
configuration.

However it would use `phpunit.xml.dist` as well if `phpunit.xml` is not
available.

The reason is to track `phpunit.xml.dist` in your repo. And to ignore
`phpunit.xml`. By default everyone (including a CI) would use
`phpunit.xml.dist` except you add `phpunit.xml` locally.

`phpunit.xml.dist` has the same XML structure as `phpunit.xml`. So it should
be detected as XML by linguist.

Example: https://github.com/erusev/parsedown/blob/master/phpunit.xml.dist

I don't know why linguist is not detecting this file as XML since it starts
with `<?xml`. Perhaps it is another issue.