Use LIBXML_VERSION instead of LIBXML_LOADED_VERSION

[provokateurin]

Summary

New error with psalm 5.

With LIBXML_LOADED_VERSION I get Cannot perform a numeric operation with a non-numeric type '21004-GITv2.10.4'. Looking at https://www.php.net/manual/en/libxml.constants.php it's not even documented...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)