Set ostruct as a gem dependency

In Ruby 3.4, requiring ostruct without pulling it in as a gem brings up a deprecation warning. Ostruct is used in the cli and sidekiq load check.

```
$ bin/bundle exec honeybadger deploy --environment=production --revision=... [email protected]:.../....git'
/home/user/.rbenv/versions/3.4.1/lib/ruby/gems/3.4.0/gems/honeybadger-5.26.1/lib/honeybadger/cli/exec.rb:8: 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.
```

Added ostruct as a runtime dependency of the gem which should appease the warning.

honeybadger.gemspec

@@ -36,4 +36,5 @@ Gem::Specification.new do |s|
   s.executables << 'honeybadger'
 
   s.add_dependency 'logger'
+  s.add_dependency 'ostruct'
 end