Add bigdecimal gem dependency for Ruby 3.4

  /opt/hostedtoolcache/Ruby/3.4.2/x64/lib/ruby/gems/3.4.0/gems/ruby-kafka-1.5.0/lib/kafka/protocol/record_batch.rb:1:
  warning: bigdecimal was loaded from the standard library, but is not
  part of the default gems starting from Ruby 3.4.0.  You can add
  bigdecimal to your Gemfile or gemspec to silence this warning.

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

fluent-plugin-kafka.gemspec

@@ -19,6 +19,10 @@ Gem::Specification.new do |gem|
   gem.add_dependency "fluentd", [">= 0.10.58", "< 2"]
   gem.add_dependency 'ltsv'
   gem.add_dependency 'ruby-kafka', '>= 1.5.0', '< 2'
+
+  # gems that aren't default gems as of Ruby 3.4
+  gem.add_dependency("bigdecimal", ["~> 3.1"])
+
   gem.add_development_dependency "rake", ">= 0.9.2"
   gem.add_development_dependency "test-unit", ">= 3.0.8"
   gem.add_development_dependency "test-unit-rr", "~> 1.0"