io.c: use PRI_OFFT_PREFIX for off_t
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55009 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
io.c: use PRI_OFFT_PREFIX for off_t
PRI_OFF_T_PREFIX to format off_t properly on Cygwin.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55009 b2dd03c8-39d4-4d8f-98ff-823fe69b080e