textproc/libxml2: bump PORTREVISION to force rebuild

FreeBSD-ports-latest - for FreeBSD 15 - currently carries packages
that have the libxml2-core package as a dependency (and the
libxml2-core).

Bump PORTREVISION to force the package builder to resynch onto ports
where there is no libxml2-core package, to reduce confusion.

(2026Q1 is unaffected, it did not have this change.)