crypto: af_alg - Add dependency on NET

Add missing dependency on NET since we require sockets for our
interface.

Should really be a select but kconfig doesn't like that:

net/Kconfig:6:error: found recursive dependency: NET -> NETWORK_FILESYSTEMS -> AFS_FS -> AF_RXRPC -> CRYPTO -> CRYPTO_USER_API_HASH -> CRYPTO_USER_API -> NET

Reported-by: Zimny Lech <napohybelskurwysynom2010@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>