Install php7-xml to avoid utf encode/decode issue

1. Try to run a local instance of the Dokuwiki installer, using PHP 7's build-in dev server.
2. Get these errors:
   1. PHP function utf8_encode is not available. Maybe your hosting provider disabled it for some reason?
   2. PHP function utf8_decode is not available. Maybe your hosting provider disabled it for some reason?

Use your package manager to install the “php7-xml” package.