crypto: powerpc - CRYPT_CRC32C_VPMSUM should depend on ALTIVEC

The optimised crc32c implementation depends on VMX (aka. Altivec)
instructions, so the kernel must be built with Altivec support in order
for the crc32c code to build.

Fixes: 6dd7a82c

 ("crypto: powerpc - Add POWER8 optimised crc32c")
Acked-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>