readline/extconf.rb: rl_hook_func_t

* ext/readline/extconf.rb (rl_hook_func_t): check pointer type.
  [ruby-dev:48089] [Bug #9702]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45518 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-04-05

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45519 b2dd03c8-39d4-4d8f-98ff-823fe69b080e