net: net_namespace: Fix undefined member in key_remove_domain()

The key_domain member in struct net only exists if we define CONFIG_KEYS.
So we should add the define when we used key_domain.

Fixes: 9b242610 ("keys: Network namespace domain tag")
Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>