DH: add internal dh_get_method()

DH: add internal dh_get_method()

This should have been publically present a long time ago, to be
consistent with the RSA, DSA and EC_KEY APIs.  However, since we've
now deprecated that kind of function for the other key types, there's
no point in adding a public function, but we still need it internally.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11193)