Add drb to gemspec

[ayunker]

When running flatware on ruby 3.3.x, we get a warning that drb will be removed from the stdlib starting in 3.4.0 and to contact the author of flatware to add it to the gemspec. Figured I could do one better and add it myself 😄.

% bundle exec rake spec
/Users/bear/src/oss/flatware/lib/flatware/sink.rb:1: warning: drb/drb is found
in drb, which will no longer be part of the default gems since Ruby 3.4.0. 
Add drb to your Gemfile or gemspec.