ossl.c: suppress warning

* ext/openssl/ossl.c (Init_ossl_locks): to suppress shorten-64-to-32
  warning, re-apply part of r41879.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42149 b2dd03c8-39d4-4d8f-98ff-823fe69b080e