improvement: move /usr/bin/screen to SGID whitelisting

see investigation here: https://github.com/TelekomLabs/chef-os-hardening/issues/5

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
Merge pull request #14 from TelekomLabs/whitelist-screen

improvement: move /usr/bin/screen to SGID whitelisting