Bug#921952: Don't include in buster without proper commitment to update in stable

[Xavier]

Hi all,

Some fixes proposed in
https://salsa.debian.org/sass-team/libsass/merge_requests/1 :
CVE-2018-19827, CVE-2019-6283, CVE-2019-6284 and CVE-2019-6286

Cheers,
Xavier