speed: add ecdhx448 to ecdh choices

CLA: trivial
Signed-off-by: Steven Noonan <steven@uplinklabs.net>

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5551)