openssl: Add conditional macros around SHA_CTX for AWS-LC

[geedo0]

AWS-LC is an BoringSSL-based libcrypto implementation. SHA_CTX is declared with the hash data specified as an array rather than as a field in upstream OpenSSL. Since AWS-LC builds against C99, we are unable to handle this with anonymous unions like BoringSSL. The workaround I propose is to add these conditional macros around the accessors within openssl_sha1_prf. After this change, everything builds successfully with AWS-LC headers.

Related to Issue #1907