Add ostruct as dependency to gemspec

Starting with ruby 3.4 ostruct will not be shipped with ruby by default.
To avoid dependency issues we add ostruct as a dependency to our
gemspec.

disposable.gemspec

@@ -19,6 +19,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "declarative",         ">= 0.0.9", "< 1.0.0"
   # spec.add_dependency "declarative-builder"  #, ">= 0.2.0"
   spec.add_dependency "representable",       ">= 3.1.1", "< 4"
+  spec.add_dependency "ostruct",             "< 1"
 
   spec.add_development_dependency "bundler"#, "~> 1.3"
   spec.add_development_dependency "rake"