FIX: cache_critical_dns was erroring without IPAddr

* sometimes cache_critical_dns would error out since "IPAddr" was
  undefined
* sometimes it autoloaded, so no error

script/cache_critical_dns

@@ -1,6 +1,7 @@
 #!/usr/bin/env ruby
 # frozen_string_literal: true
 
+require 'ipaddr'
 require 'resolv'
 require 'time'
 require 'socket'