Explicitly require ostructn.gemspec

ref..rbenv/versions/3.4.1/lib/ruby/gems/3.4.0/gems/gon-6.4.0/lib/gon/base.rb:1: warning: ostruct 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.

closes gazay#270

gon.gemspec

@@ -20,6 +20,9 @@ Gem::Specification.new do |s|
   s.add_dependency 'i18n', '>= 0.7'
   s.add_dependency 'request_store', '>= 1.0'
   s.add_dependency 'multi_json'
+  if RUBY_VERSION >= '3.5.0'
+    s.add_dependency 'ostruct'
+  end
   s.add_development_dependency 'rabl', '0.11.3'
   s.add_development_dependency 'rabl-rails'
   s.add_development_dependency 'rspec', '>= 3.0'