Fix ostruct LoadError in struct_vs_ostruct benchmark

The ostruct default gem became bundled in Ruby v4.0 [1]. It needs to be added
to the Gemfile to allow the `benchmarks/struct_vs_ostruct.rb` standalone script
to run, but it doesn't need to be added as a dependency in the gemspec.

[1]: https://rubyreferences.github.io/rubychanges/4.0.html#default-gems-that-became-bundled

Author: floehopper

Gemfile

@@ -6,6 +6,7 @@ group :development do
     gem 'benchmark'
     gem 'irb'
     gem 'logger'
+    gem 'ostruct'
   end
   gem 'rspec', '>= 3.11.0'
   gem 'rake'