crypto: null - Add missing Kconfig tristate for NULL2

This patch adds a missing tristate statement to Kconfig for the
new CRYPTO_NULL2 option.

Fixes: 149a3971

 ("crypto: aead - Add type-safe geniv init/exit helpers")
Reported-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>