Fix building docs

[Jakuje]

Description

This was accidentally removed in 0a96cb5

Checklist

• This is not a code change

Reviewer's checklist:

• Any issues marked for closing are fully addressed
• There is a test suite reasonably covering new functionality or modifications
• This feature/change has adequate documentation added
• A changelog entry is added if the change is significant
• Code conform to coding style that today cannot yet be enforced via the check style test
• Commits have short titles and sensible text
• Doc string are properly updated

[simo5]

Actually using openssl-sys may allow us to enable docs building for crates.io as well, so far it did not work because I did not have a way to guarantee openssl was available, but they have a specific exception for openssl-sys IIRC.
Do you want to tackle enabling that as well ?
See #442 for context

[Jakuje]

I agree that would be good to have a generic solution, but this is just a bandaid and fixup of what I messed (and missed) in 0a96cb5 where I removed this block as no longer needed by changing default to openssl-sys, but did not add it back when we reverted.

This is currently preventing the docs of latest release to build: https://docs.rs/crate/ossl/latest

[simo5]

LGTM