Fix Ruby 3.3.5 ostruct warning

[yenshirak]

The following warning is thrown with Ruby 3.3.5:

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 koala-3.6.0 to request adding ostruct into its gemspec.

• The PR is based on the most recent master commit and has no merge conflicts.

[ylecuyer]

Thanks for the contribution, I'm not sure if it is worth doing a release right now with just this fix as this is a warning atm. As it will break only later with ruby 3.5 they will probably be another release before