Add webrick as developement dependency

Because Ruby 3.0 does not bundle webrick as default gem. Signed-off-by: Hiroshi Hatake <cosmo0920.oucc@gmail.com>

Author: cosmo0920

fluent-plugin-kafka.gemspec

@@ -21,4 +21,5 @@ Gem::Specification.new do |gem|
  gem.add_dependency 'ruby-kafka', '>= 1.2.0', '< 2'
  gem.add_development_dependency "rake", ">= 0.9.2"
  gem.add_development_dependency "test-unit", ">= 3.0.8"
+ gem.add_development_dependency "webrick"
 end