Add ostruct to gemfile

[afdev82]

Hello,

I am getting this warning in my console logs:

warning: /home/antonio/.rbenv/versions/3.3.5/lib/ruby/3.3.0/ostruct.rb 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.
Also please contact the author of cached_resource-9.0.0 to request adding ostruct into its gemspec.

If it's fine, I could submit a PR with that change.