Add ostruct to gemspec for Ruby 3.5

[halilim]

Fixes the warning:

.../lib/whois/parser/super_struct.rb:1: warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add ostruct to your Gemfile or gemspec to silence this warning.

[jarthod]

Thanks for this @halilim, for the record this project isn't really maintained by @weppos any more, so I ended up starting my fork in which I fixed most of the issues. I try to keep it maintained in hope it could be merged back upstream some day so everyone can benefit from it. I just cherry-picked your PR here jarthod@66c5b03 and updated the test suite here: https://github.com/jarthod/whois-parser/actions/runs/12810390972 (it's now passing even on ruby-head). So if you need this (and the many other fixes) you can use my fork in the meantime:

gem 'whois-parser', github: 'jarthod/whois-parser' # Unofficial but more up-to-date fork, check status at https://github.com/jarthod/whois-parser