editors/abiword: Fix build with libxml2 2.12 and newer

Backport upstream commit 2a06be6a10a0718f8a3d8e00c317f5042c99a467

Reference:
https://gitlab.gnome.org/World/AbiWord/-/commit/2a06be6a10a0718f8a3d8e00c317f5042c99a467

PR: 280157
Approved by: portmgr (maintainer timeout, 2+ weeks)