Mark CVE -2018-19205 as ignored for stretch

The respective feature would rely on a properly working php-crypt-gpg
installation. Those might be custom on the system (as php-crypt-gpg is
not in stretch). But plugins/enigma/lib/enigma_driver_gnupg.php from
1.2.x has bigger issues anyway, since it's decrypt() function doesn't
verify signatures on encrypted message.