Workaround for YARD's warning

This is a workaround to prevent the following warning in YARD:

```console
$ ruby -v
ruby 3.4.0dev (2024-04-19T07:33:16Z master 7951b349ab) [x86_64-darwin23]
$ cd path/to/rubocop
$ bundle exec rake
/Users/koic/.rbenv/versions/3.4-dev/lib/ruby/gems/3.4.0+0/gems/yard-0.9.36/lib/yard/templates/template_options.rb:2:
warning: ostruct was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.5.0.
Add ostruct to your Gemfile or gemspec.
```

The fundamental resolution will likely be addressed in lsegal/yard#1545.

Gemfile

@@ -8,6 +8,10 @@ gem 'asciidoctor'
 gem 'bump', require: false
 gem 'bundler', '>= 1.15.0', '< 3.0'
 gem 'memory_profiler', platform: :mri
+# FIXME: This is a workaround to prevent the following warning in YARD:
+# https://github.com/lsegal/yard/pull/1545
+# Please remove this dependency when the issue is resolved.
+gem 'ostruct'
 # FIXME: This is a workaround for incompatibilities between Prism 0.25.0 and 0.26.0.
 # To upgrade to Prism 0.26+, it is necessary to investigate the following build error
 # and provide feedback to Prism: