win32ole.c: suppress warning

* ext/win32ole/win32ole.c (ole_rec2variant): cast to suppress a
  format warning on x86_64-cygwin.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46981 b2dd03c8-39d4-4d8f-98ff-823fe69b080e