iconv: detect and make curses depend on it

curses will use it for proper wide output support.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-Id: <20190311135127.2229-2-samuel.thibault@ens-lyon.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>