Add dependency on VSX as otherwise the build will fail without
it.

Fixes: 161fca7e ("crypto: powerpc - Add chacha20/poly1305-p10 to Kconfig and Makefile")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308251906.SYawej6g-lkp@intel.com/Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>