Move `rb_sys` gem dep to gemspec

[georgeclaghorn]

Rubygems can't see dependencies in Gemfiles. Like the generated comment at the top of the Gemfile suggests, gem dependencies intended to be exposed to users and automatically installed with the gem must be listed in the gemspec.

Rubygems.org shows no runtime dependencies for tiktoken_ruby:

Compare to another gem with a Rust native extension:

Fixes #1.

[jpr5]

I second this. Dependency should be required explicitly so bundler can do the right thing.