OpenSSL_add_ssl_algorithm-is-deprecated() is deprecated, make it so

This function is documented to be deprecated since OpenSSL 1.1.0.  We
need to make it so in openssl/ssl.h as well.

Fixes #6565

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6588)