textproc/libxml2: fix build with VALIDATION option off

Grab two commits from upstream that fix the build when
--no-valid is passed to configure; in the ports build
that translates to the VALIDATION option turned off.
These come straight from GNOME GitLab, although I've
bunged them into one patch-* file.

Not bumping PORTREVISION, since with this option off
it would never have built, and with the option on
the resulting package is unchanged.

PR: 253596