Update the vendored copy of libexpat to 2.4.1 (from 2.2.8) to get the
fix for the CVE-2013-0340 "Billion Laughs" vulnerability. This copy
is most used on Windows and macOS.
Co-authored-by: Łukasz Langa <lukasz@langa.pl>
with it a vareity of bug fixes, both security and behavior. See
http://www.libexpat.org/ for the list.
NOTE: I already backported the expat hash randomization fix in March.
Fixes issue #14340.