crypto: null - Use NULL2 in Makefile

The Kconfig option NULL2 has been added as CRYPTO_MANAGER now
depends indirectly on NULL2.  However, the Makefile was not updated
to use the new option, resulting in potential build failures when
only NULL2 is enabled.

Fixes: 149a3971 ("crypto: aead - Add type-safe geniv init/exit helpers")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/Makefile

@@ -46,7 +46,7 @@ obj-$(CONFIG_CRYPTO_CMAC) += cmac.o
 obj-$(CONFIG_CRYPTO_HMAC) += hmac.o
 obj-$(CONFIG_CRYPTO_VMAC) += vmac.o
 obj-$(CONFIG_CRYPTO_XCBC) += xcbc.o
-obj-$(CONFIG_CRYPTO_NULL) += crypto_null.o
+obj-$(CONFIG_CRYPTO_NULL2) += crypto_null.o
 obj-$(CONFIG_CRYPTO_MD4) += md4.o
 obj-$(CONFIG_CRYPTO_MD5) += md5.o
 obj-$(CONFIG_CRYPTO_RMD128) += rmd128.o