Add ostruct as runtime dependency

The ostruct gem moves from default gems to a normal gem in Ruby 3.5. It
raises the following warning on Ruby 3.3.5:

```
lib/puppet/util/command_line.rb:14: warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
```

puppet.gemspec

@@ -30,6 +30,7 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency('puppet-resource_api', '~> 1.5')
   spec.add_runtime_dependency('scanf', '~> 1.0')
   spec.add_runtime_dependency('semantic_puppet', '~> 1.0')
+  spec.add_runtime_dependency('ostruct', '~> 0.6.0')
 
   platform = spec.platform.to_s
   if platform == 'universal-darwin'