src: fix FIPS section in Sign::SignFinal

Currently, while FIPS is not supported yet for this release there might
be an option to dynamically link against a FIPS compatible OpenSSL
version.

This commit fixes the compiler errors.

PR-URL: https://github.com/nodejs/node/pull/25412


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>