lint: allow Beerware and XSkat licenses
These licenses were already in our repo. The are not FSF- or OSI-approved, but they are included in Debian. So for now at least, we should maintain them in our list until we have a clearer picture of how we should make the list of licenses we support.
- !682 (merged)
- Beerware in Debian: https://metadata.ftp-master.debian.org/changelogs//main/libm/libmd/libmd_1.0.1-3_copyright
- XSkat in Debian: https://metadata.ftp-master.debian.org/changelogs//main/x/xskat/xskat_4.0-7_copyright