[ruby/irb] Make it possible to use measure even if it is installed as gem

Include the file that implements this command in the irb gem.

Co-Authored-By: scivola <scivola@users.noreply.github.com>

https://github.com/ruby/irb/commit/d8dfea5b57

lib/irb/irb.gemspec

@@ -33,6 +33,7 @@ Gem::Specification.new do |spec|
     "lib/irb/cmd/fork.rb",
     "lib/irb/cmd/help.rb",
     "lib/irb/cmd/load.rb",
+    "lib/irb/cmd/measure.rb",
     "lib/irb/cmd/nop.rb",
     "lib/irb/cmd/pushws.rb",
     "lib/irb/cmd/subirb.rb",