crypto: sm2 - Provide sm2_compute_z_digest when sm2 is disabled

When sm2 is disabled we need to provide an implementation of
sm2_compute_z_digest.

Fixes: e5221fa6 ("KEYS: asymmetric: Move sm2 code into x509_public_key")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306231917.utO12sx8-lkp@intel.com/


Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>