Add ostruct to gemspec

[Havish96]

Getting this warning:

tmp/build_29022407/vendor/ruby-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.

Request to add ostruct to Gemfile or gemspec to silence this warning.

Thank you.

[Havish96]

Thanks 🙌