crypto: null - Add missing Kconfig tristate for NULL2

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

Fixes: 149a39717dcc ("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>

crypto/Kconfig

@@ -157,6 +157,7 @@ config CRYPTO_NULL
 	  These are 'Null' algorithms, used by IPsec, which do nothing.
 
 config CRYPTO_NULL2
+	tristate
 	select CRYPTO_ALGAPI2
 	select CRYPTO_BLKCIPHER2
 	select CRYPTO_HASH2