devel/tclxml: fix build with libxml2 >= 2.12

Version 2.12 of libxml2 introduced a backwards incompatible change in
the signature of some functions, notably the second argument to
xmlStructuredErrorFunc has gained a const qualifier, changing from
xmlError *error to const xmlError *error.

To make the signatures compatible with both pre- and post-2.12, I have
introduced a check on LIBXML_VERSION.

PR: 279968
Reported by: dizzy