spec: add requirement to uucp
Since recently uucp group and user are not created by setup package
as well as /var/spool/uucp is no longer created by filesystem package
(#961952). mgetty however still might need those, but we don't want to
bother with them majority of users. Modem users are quite rare nowadays.
Strictly speaking it is possible to use mgetty without uucp, having
the package on the system prevents warnings mgetty prints about
non-existing uucp user and group. Secondly, odds are high that mgetty
user is also uucp user.