Undocument linux-*-expat vulnerabilities.

linux-*-expat is only used by linux-*-fontconfig to read configuration
files written in XML and by dbus-binding-tool(1) from linux-*-dbus-glib, a
development tool that generates C code from an Introspection XML file to
expose a GObject via D-Bus.

These vulnerabilities are therefore not believed to be exploitable on
FreeBSD and only cause annoying warnings and prevent installation of
linux-*-expat. It also does not look like Red Hat will provide fixes for
these any time soon.

PR: 210155