Add ostruct dependency to fix warning on Ruby 3.3.5+ (#542)

Fixes the following warning on Ruby 3.3.5+ and Ruby 3.4:

> sshkit-1.23.0/lib/sshkit.rb:29: 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.

Closes #541

sshkit.gemspec

@@ -24,6 +24,7 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency('net-ssh',  '>= 2.8.0')
   gem.add_runtime_dependency('net-scp',  '>= 1.1.2')
   gem.add_runtime_dependency('net-sftp', '>= 2.1.2')
+  gem.add_runtime_dependency('ostruct') if RUBY_VERSION >= "2.5"
 
   gem.add_development_dependency('danger')
   gem.add_development_dependency('minitest', '>= 5.0.0')