Add rdoc as an explicit dependency for Ruby 4.0 compatibility

Ruby 4.0 no longer ships rdoc as a default gem, so `require "rdoc/task"`
fails under `bundle exec` unless rdoc is listed in the Gemfile.

Author: ledsun

Gemfile

@@ -8,6 +8,7 @@ group :development do
   gem "rake"
   gem "rake-compiler"
   gem "rb_sys", "0.9.108"
+  gem "rdoc", "~> 7.0"
 end
 
 group :check do