Fix building without scrypt

Building without the scrypt KDF is now possible, the OPENSSL_NO_SCRYPT
define is honored in code. Previous this lead to undefined references.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Stephen Henson <steve@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4116)