Update information for CVE-2018-1000024/squid3

The Debian builds do Build-Depends on libexpat1-dev and libxml2-dev for
ESI support since 3.1.0.14-2. The CVE-2018-1000024 problem is limited to
Squid custom ESI parser, which vulnerable is present, but since Squid is
built to use libxml2 or the libexpat XML parsers it does not have the
problem in the resulting binary package. Mark it thus as unimportant.