Add ostruct as runtime dependency

Until Ruby 4 ostruct was included in the vendored gems. We now need to
explicitly list it.

Author: bastelfreak

openfact.gemspec

@@ -50,5 +50,6 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency 'benchmark', '< 0.6'
   spec.add_runtime_dependency 'hocon', '~> 1.3'
   spec.add_runtime_dependency 'logger', '~> 1.5'
+  spec.add_runtime_dependency 'ostruct', '< 0.7'
   spec.add_runtime_dependency 'thor', ['>= 1.0.1', '< 2']
 end