libcurl: fix typo in configure option w/o OpenSSL

When not using OpenSSL, the correct option to configure is --without-ssl
with two dashes.

Fixes: b8b78e7e ("libcurl: Allow selection of TLS package libcurl will use")

Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>