rmd160/extconf.rb: fix function name

* ext/digest/rmd160/extconf.rb: fix transform function name to
  check.  [ruby-core:65091] [Bug #10252]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47801 b2dd03c8-39d4-4d8f-98ff-823fe69b080e