feature: add frozen string magic comment

puppetlabs-augeas_core and puppet-augeasproviders_core both provide the
augeas feature.  However the both have slightly different files (this
lacking the frozen strings magic comment.  This causes puppet to perform
a change on every puppet run.

This PR updates the feature in this repo to add the magic comment so
both files match.  More then happy to explore alternate fixes

lib/puppet/feature/augeas.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'puppet/util/feature'
 
 Puppet.features.add(:augeas, libs: ['augeas'])