fix(gemspec): add ostruct as a development dependency (#1778)

```
❯ RUBYOPT=-W:deprecated bundle exec rspec
/Users/ydah/factory_bot/spec/acceptance/sequence_spec.rb:3: 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.

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750 examples, 0 failures
```

Author: ydah

Gemfile.lock

@@ -84,6 +84,7 @@ GEM
     minitest (5.25.4)
     multi_test (1.1.0)
     mutex_m (0.3.0)
+    ostruct (0.6.3)
     parallel (1.26.3)
     parser (3.3.7.0)
       ast (~> 2.4.1)
@@ -168,6 +169,7 @@ DEPENDENCIES
   cucumber
   factory_bot!
   mutex_m
+  ostruct
   rake
   rspec
   rspec-its

factory_bot.gemspec

@@ -33,6 +33,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency("aruba")
   s.add_development_dependency("cucumber")
   s.add_development_dependency("mutex_m")
+  s.add_development_dependency("ostruct")
   s.add_development_dependency("rake")
   s.add_development_dependency("rspec")
   s.add_development_dependency("rspec-its")