fix: Add benchmark dependency to remove warning

Author: jsonbailey

launchdarkly-server-sdk.gemspec

@@ -35,6 +35,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rubocop", "~> 1.37"
   spec.add_development_dependency "rubocop-performance", "~> 1.15"
 
+  spec.add_runtime_dependency "benchmark", "~> 0.4"
   spec.add_runtime_dependency "semantic", "~> 1.6"
   spec.add_runtime_dependency "concurrent-ruby", "~> 1.1"
   spec.add_runtime_dependency "ld-eventsource", "2.2.3"