console.c: supppress warnings

* ext/io/console/console.c (console_iflush, console_oflush): supppress
  unused-but-set-variable warnings on no tcflush.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39778 b2dd03c8-39d4-4d8f-98ff-823fe69b080e