Fix detection of OpenSSL 1.1.1 or later in curl_sha512_256.c

[pghmcfc]

Use the same OPENSSL_VERSION_NUMBER comparison as in lib/vtls/openssl.c.

As it stands, builds using an OpenSSL 1.1.0 release fail due to trying to use the unavailable symbol EVP_sha512_256.

[bagder]

/cc @Karlson2k

[Karlson2k]

I made detection according to the OpenSSL documentation.
If the symbol is really missing I think the fix is correct.
As libcurl has the plan B (the local version), it's safer to merge this PR as in the worst case it would just lower the performance.

[pghmcfc]

The version number would be correct as per the current numbering documentation. However, this changed with OpenSSL 3.x - the documentation for OpenSSL 1.x was slightly different: https://www.openssl.org/docs/man1.1.1/man3/OPENSSL_VERSION_NUMBER.html

[bagder]

Thanks!