unbound: avoid calling deprecated OpenSSL function

SSL_CTX_set_ecdh_auto is deprecated and has no effect (for reference see
https://github.com/openssl/openssl/commit/2ecb9f2d18614fb7b7b42830a358b7163ed43221).

As unbound's config.h is manually maintained just turn off
HAVE_DECL_SSL_CTX_SET_ECDH_AUTO so that the deprecated function won't
be called.

Reviewed by: ngie
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40341