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>