Add ostruct to gemspec dependencies

This gem was historically included in the default gemset. Starting with
Ruby 3.3.5 though, requiring `ostruct` without explicitly adding it as
dependency generates the following warning:

    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.

grape-swagger-rails.gemspec

@@ -16,6 +16,7 @@ Gem::Specification.new do |spec|
   spec.files         = `git ls-files`.split($INPUT_RECORD_SEPARATOR)
   spec.require_paths = %w[lib]
   spec.add_dependency 'railties', '>= 6.0.6.1'
+  spec.add_dependency 'ostruct'
   spec.metadata = {
     'bug_tracker_uri' => 'https://github.com/ruby-grape/grape-swagger-rails/issues',
     'changelog_uri' => 'https://github.com/ruby-grape/grape-swagger-rails/blob/master/CHANGELOG.md',