Add ostruct to Gemfile for Ruby >= v3.4.0

I was seeing the following warning when I ran `bundle exec rake lint`
locally with Ruby v3.4.0-rc1:

    ~/.asdf/installs/ruby/3.4.0-rc1/lib/ruby/gems/3.4.0+1/gems/rubocop-0.58.2/lib/rubocop.rb:607:
    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.

Gemfile

@@ -18,6 +18,11 @@ if RUBY_VERSION >= '3.3.0'
   gem 'base64'
 end
 
+# Avoid ostruct gem warning about it not being available in Ruby v3.5
+if RUBY_VERSION >= '3.4.0'
+  gem 'ostruct'
+end
+
 if RUBY_ENGINE == 'jruby'
   # Workaround for https://github.com/jruby/jruby/issues/8488
   gem 'jar-dependencies', '~> 0.4.1'