Suppress -Warray-parameter warnings in pgcrypto/sha2.c.

Suppress -Warray-parameter warnings in pgcrypto/sha2.c.

This is exactly the same problem as commit 1b242f42b fixed in ecpg,
but in contrib/pgcrypto.  Commit 273c458a2 eliminated the problem
here for v10 and up.  We hadn't noticed for exactly the same reasons
enumerated in bbbf22cf3.

Back-patch down to 9.2, pursuant to newly-established project policy
about keeping out-of-support branches buildable.

Discussion: https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b@enterprisedb.com