CI: The "cross-build" job doesn't have EVP_MD_CTX_new

[Dan Drown]

I assume it's from this change: https://gitlab.com/NTPsec/ntpsec/-/commit/7c8b5fe20eafe911be4eac64467a032a52753313#887b5ad6972aa02f9a0434c248cd872a994e6fc5_44_41

Ubuntu 16.04 is pretty old, does it need to be supported?
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