debian/control: Depend on lsb-core.

In the past few days several people on IRC have had issues with Facter
not filling in lsbdistcodename which can usually be traced back to one
of the lsb-* packages missing on Debian.

By depending on lsb-core we ensure that core LSB functionality is there.
This package additionally depends on lsb-base, lsb-release and
lsb-security ensuring everything is in place for Facter.