chore: Add 'ostruct' to Gemfile

[yahonda]

This pull request adds 'ostruct' to Gemfile to suppress warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0..

Steps to reproduce

$ ruby -v
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [x86_64-linux]
$ bundle exec rspec -w

Warning fixed by this commit

/path/to/google-api-ruby-client/google-apis-core/spec/spec_helper.rb:27: warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.