Fix facter error on non-Linux operating systems

They do not have the 'distro' key in the 'os' fact, which caused an error when Puppet attempted determine whether to use the provider:

error while resolving custom facts in /var/puppet/lib/facter/package_provider.rb: Could not autoload puppet/provider/package/xbps: undefined method `[]' for nil:NilClass

One OS where this occured is FreeBSD.