crypto: ixp4xx - Select CRYPTO_AUTHENC

Without CRYPTO_AUTHENC the driver fails to build:
drivers/built-in.o: In function `ixp_module_init':
ixp4xx_crypto.c:(.init.text+0x3250): undefined reference to `crypto_aead_type'

Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>