osdetection: add PostmarketOS

[Hyask]

Running Lynis master on my Pinephone, it failed to detect the OS. Indeed, PostmarketOS was not yet included in the OS detection logic.

Here is the /etc/os-release for reference.

PRETTY_NAME="postmarketOS v21.12.3"
NAME="postmarketOS"
VERSION_ID="v21.12.3"
VERSION="v21.12.3"
ID="postmarketos"
ID_LIKE="alpine"
HOME_URL="https://www.postmarketos.org/"
SUPPORT_URL="https://gitlab.com/postmarketOS"
BUG_REPORT_URL="https://gitlab.com/postmarketOS/pmaports/issues"

Let me know if I missed anything, or if more testing is required.