crypto: vmx - Fix warning on p8_ghash_alg

The compiler complains that p8_ghash_alg isn't declared which is
because the header file aesp8-ppc.h isn't included in ghash.c.
This patch fixes the warning.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>