Add explicit dependency on ostruct

[bmulholland]

Ruby 3.3.5 outputs:

warning: lib/ruby/3.3.0/ostruct.rb was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add ostruct to your Gemfile or gemspec to silence this warning.
Also please contact the author of nylas-6.1.1 to request adding ostruct into its gemspec.

[mrashed-dev]

Thanks for the heads up @bmulholland !