readline/extconf.rb: fix typo

* ext/readline/extconf.rb: fix typo, `$defs` not `$DEFS`.
  [ruby-core:61756] [Bug #9578]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45488 b2dd03c8-39d4-4d8f-98ff-823fe69b080e