security/openconnect: Fix build with libxml 2.12.0 and newer
Backport upstream commit 6c83dfd2e1cfb9539cd527424910298a52c6d11f
Reference:
https://gitlab.com/openconnect/openconnect/-/commit/6c83dfd2e1cfb9539cd527424910298a52c6d11f
PR: 279761
Reviewed by: zi (maintainer)