extconf.rb: rl_hook_func_t for old readline

* ext/readline/extconf.rb (rl_hook_func_t): define as Function for
  very old readline versions.  [ruby-core:61209] [Bug #9578]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45240 b2dd03c8-39d4-4d8f-98ff-823fe69b080e