crypto: zstd - replace zero-length array with flexible array member

Replace the deprecated zero-length array with a modern flexible array
member in the struct zstd_ctx.

No functional changes intended.

Link: https://github.com/KSPP/linux/issues/78


Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>