Make data argument const in SSL_dane_tlsa_add

The data argument of SSL_dane_tlsa_add is used read-only, so it
should be const.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5056)