Fixed typo in enc.c warning

"warning: iv not use by this cipher" -> "warning: iv not used by this cipher"
CLA: trivial

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8608)

(cherry picked from commit 9c119bc6)