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.

* fdroidserver!682
* https://github.com/spdx/license-list-XML/issues/876
* 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

fdroidserver/lint.py

@@ -657,6 +657,7 @@ APPROVED_LICENSES = [
     'Artistic-1.0-Perl',
     'Artistic-1.0-cl8',
     'Artistic-2.0',
+    'Beerware',
     'BSD-2-Clause',
     'BSD-2-Clause-FreeBSD',
     'BSD-2-Clause-Patent',
@@ -788,6 +789,7 @@ APPROVED_LICENSES = [
     'X11',
     'XFree86-1.1',
     'Xnet',
+    'XSkat',
     'YPL-1.1',
     'ZPL-2.0',
     'ZPL-2.1',