Add explicit ostruct dependency to fix warning

[zachmargolis]

• ostruct will not be loaded by default in Ruby 3.5

Example warning:

/srv/idp/shared/bundle/ruby/3.4.0/bundler/gems/saml_idp-bdf8e1f93707/lib/saml_idp/configurator.rb:1: warning: /opt/ruby_build/versions/3.4.1/lib/ruby/3.4.0/ostruct.rb was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.