Add dependency of ostruct gem, to fix warning of ostruct was loaded (#363)

Author: taketo1113

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## Unreleased
+
+* Fix warning: ostruct was loaded from the standard library ([#363](https://github.com/rubyconfig/config/pull/363))
+
 ## 5.5.1
 
 ### Documentation

config.gemspec

@@ -28,6 +28,7 @@ Gem::Specification.new do |s|
   s.required_ruby_version = '>= 2.6.0'
 
   s.add_dependency 'deep_merge', '~> 1.2', '>= 1.2.1'
+  s.add_dependency 'ostruct'
 
   s.add_development_dependency 'dry-validation', *Config::DryValidationRequirements::VERSIONS
   s.add_development_dependency 'rake', '~> 12.0', '>= 12.0.0'