textproc/libxml2: add upstream fix for CVE-2021-3541

This is relapted to parameter entities expansion and following
the line of the billion laugh attack. Somehow in that path the
counting of parameters was missed and the normal algorithm based
on entities "density" was useless.

PR: 256094
Obtained from: https://gitlab.gnome.org/GNOME/libxml2/-/commit/8598060bacada41a0eb09d95c97744ff4e428f8e
Security: CVE-2021-3541