textproc/php81-xml: fix compatibility with libxml2>=2.13

Fix compatibility issue with libxml2>=2.13 which caused parse error
on some XML files so that devel/pear work again with php81.

See https://github.com/php/php-src/issues/14834 for details.

Approved by: portmgr blanket (backport trivial fix from upstream)

(cherry picked from commit a8592eb7375ea088a96051ad62e46af739f23331)