X.509: Fix modular build of public_key_sm2

The sm2 code was split out of public_key.c in a way that breaks
modular builds.  This patch moves the code back into the same file
as the original motivation was to minimise ifdefs and that has
nothing to do with splitting the code out.

Fixes: 21552563 ("X.509: support OSCCA SM2-with-SM3...")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>