Add ostruct to dependencies in preparation of Ruby 3.5.0

[matthewbjones]

After upgrading a project to Ruby version 3.3.5, we see the following warnings:

/Users/mjones/.rvm/gems/ruby-3.3.5@redacted-project-name/gems/rdf-3.3.2/lib/rdf.rb:5: warning: /Users/mjones/.rvm/rubies/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.
You can add ostruct to your Gemfile or gemspec to silence this warning.
Also please contact the author of rdf-3.3.2 to request adding ostruct into its gemspec.

This PR adds the ostruct library to the gemspec as suggested, which removes the warning when using the rdf gem.

[CLAassistant]

All committers have signed the CLA.

[coveralls]

coverage: 91.739%. remained the same
when pulling a3b50b7 on matthewbjones:develop
into 12aee59 on ruby-rdf:develop.