Add missing macros for DHxparams

DHparams has d2i_DHparams_fp, d2i_DHxparams_bio etc, but the equivalent
macros for DHxparams were omitted.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3111)
(cherry picked from commit ff79a244)