load.c (ruby_init_ext): make idempotent to suppress warnings

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45232 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-03-02

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45233 b2dd03c8-39d4-4d8f-98ff-823fe69b080e