Add benchmark gem, since it is not part of Ruby 4

Author: mattbrictson

sshkit.gemspec

@@ -28,6 +28,7 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency('net-sftp', '>= 2.1.2')
   gem.add_runtime_dependency('ostruct')
 
+  gem.add_development_dependency('benchmark')
   gem.add_development_dependency('danger')
   gem.add_development_dependency('minitest', '>= 5.0.0')
   gem.add_development_dependency('minitest-reporters')