[NETFILTER] ctnetlink: add module alias to fix autoloading

Add missing module alias. This is a must to load ctnetlink on demand. For
example, the conntrack tool will fail if the module isn't loaded.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>