Add support for Mageia to osfamily.rb

The addition of Mageia OS in https://github.com/puppetlabs/facter/commit/e2f040a5bc4ed7539ecc54a5c3d26f93a9ed6a18 (Ticket #9929) was incomplete. Mageia is derived from Mandrake and Mandriva, and so is in the same family.