crypto: kdf - select SHA-256 required for self-test

The self test of the KDF is based on SHA-256. Thus, this algorithm must
be present as otherwise a warning is issued.

Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>