gem: add ostruct gem as dependency for Ruby 4.0

[Watson1978]

This patch will fix following error:

$ bundle exec rake test
/home/watson/.rbenv/versions/4.0.1/bin/ruby -w -I"lib:lib:test" /home/watson/.rbenv/versions/4.0.1/lib/ruby/gems/4.0.0/gems/rake-13.3.1/lib/rake/rake_test_loader.rb "test/test_in_s3.rb" "test/test_out_s3.rb"
/home/watson/.rbenv/versions/4.0.1/lib/ruby/gems/4.0.0/gems/fluentd-1.18.0/lib/fluent/plugin_helper.rb:46: warning: method redefined; discarding old inherited
/home/watson/.rbenv/versions/4.0.1/lib/ruby/gems/4.0.0/gems/fluentd-1.18.0/lib/fluent/plugin_helper.rb:46: warning: previous definition of inherited was here
/home/watson/src/fluent-plugin-s3/lib/fluent/plugin/in_s3.rb:7: warning: CGI::Util is removed from Ruby 4.0. Please use cgi/escape instead for CGI.escape and CGI.unescape features.
If you are using CGI.parse, please install and use the cgi gem instead.
/home/watson/src/fluent-plugin-s3/test/test_in_s3.rb:14: warning: ostruct used to be loaded from the standard library, but is not part of the default gems since Ruby 4.0.0.
You can add ostruct to your Gemfile or gemspec to fix this error.
/home/watson/.rbenv/versions/4.0.1/lib/ruby/4.0.0/bundled_gems.rb:60:in 'Kernel.require': cannot load such file -- ostruct (LoadError)
Did you mean?  tsort
        from /home/watson/.rbenv/versions/4.0.1/lib/ruby/4.0.0/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
        from /home/watson/src/fluent-plugin-s3/test/test_in_s3.rb:14:in '<top (required)>'
        from /home/watson/.rbenv/versions/4.0.1/lib/ruby/4.0.0/bundled_gems.rb:60:in 'Kernel.require'
        from /home/watson/.rbenv/versions/4.0.1/lib/ruby/4.0.0/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
        from /home/watson/.rbenv/versions/4.0.1/lib/ruby/gems/4.0.0/gems/rake-13.3.1/lib/rake/rake_test_loader.rb:21:in 'block in <main>'
        from /home/watson/.rbenv/versions/4.0.1/lib/ruby/gems/4.0.0/gems/rake-13.3.1/lib/rake/rake_test_loader.rb:6:in 'Array#select'
        from /home/watson/.rbenv/versions/4.0.1/lib/ruby/gems/4.0.0/gems/rake-13.3.1/lib/rake/rake_test_loader.rb:6:in '<main>'
rake aborted!
Command failed with status (1): [ruby -w -I"lib:lib:test" /home/watson/.rbenv/versions/4.0.1/lib/ruby/gems/4.0.0/gems/rake-13.3.1/lib/rake/rake_test_loader.rb "test/test_in_s3.rb" "test/test_out_s3.rb" ]
/home/watson/.rbenv/versions/4.0.1/bin/bundle:25:in '<main>'
Tasks: TOP => test
(See full trace by running task with --trace)

We need to specify the ostruct gem as dependency because it is a bundled gem from Ruby 4.0.
Same reason with fluent/fluentd#5249

Ref:

fluent-plugin-s3/test/test_in_s3.rb

Line 14 in 9e5e9e8

require 'ostruct'

fluent-plugin-s3/test/test_out_s3.rb

Line 12 in 9e5e9e8

require 'ostruct'

[kenhys]

LGTM.

[Watson1978]

Thanks.