[ruby/readline] Fix readline-ext gem loading in non Unix-like environments

https://github.com/ruby/readline/commit/6e1e5d0eb8

lib/readline.rb

@@ -1,5 +1,5 @@
 begin
-  require 'readline.so'
+  require "readline.#{RbConfig::CONFIG["DLEXT"]}"
 rescue LoadError
   require 'reline' unless defined? Reline
   Object.send(:remove_const, :Readline) if Object.const_defined?(:Readline)