crypto: vmx - Ensure ghash-generic is enabled

Select CRYPTO_GHASH for vmx_crypto since p8_ghash uses it as the
fallback implementation.
Signed-off-by: Marcelo Cerri <marcelo.cerri@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/vmx/Kconfig

 config CRYPTO_DEV_VMX_ENCRYPT
 	tristate "Encryption acceleration support on P8 CPU"
 	depends on CRYPTO_DEV_VMX
+	select CRYPTO_GHASH
 	default m
 	help
 	  Support for VMX cryptographic acceleration instructions on Power8 CPU.