Adds 'ostruct' as a development dependency

It used to be loaded from the standard library, but is not part of the default gems since Ruby 3.5.0

Author: picandocodigo

elasticsearch/elasticsearch.gemspec

@@ -52,6 +52,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'base64'
   s.add_development_dependency 'bundler'
   s.add_development_dependency 'debug' unless defined?(JRUBY_VERSION)
+  s.add_development_dependency 'ostruct'
   s.add_development_dependency 'pry'
   s.add_development_dependency 'rake'
   s.add_development_dependency 'require-prof' unless defined?(JRUBY_VERSION) || defined?(Rubinius)