Add benchmark gem as a runtime dependency

Since Ruby 4.0.0, `benchmark` gem is not part of the default gems.
We need to add that to use on  Ruby 4.0+.

Fixes: #444

Author: y-yagi

Gemfile.lock

@@ -4,6 +4,7 @@ PATH
     packwerk (3.2.3)
       activesupport (>= 6.0)
       ast
+      benchmark
       better_html
       bundler
       constant_resolver (>= 0.3)

packwerk.gemspec

@@ -39,6 +39,7 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = ">= 3.1"
 
   spec.add_dependency("activesupport", ">= 6.0")
+  spec.add_dependency("benchmark")
   spec.add_dependency("bundler")
   spec.add_dependency("constant_resolver", ">= 0.3")
   spec.add_dependency("parallel")