netfilter: ipset: The unnamed union initialization may lead to compilation error

The unnamed union should be possible to be initialized directly, but
unfortunately it's not so:

/usr/src/ipset/kernel/net/netfilter/ipset/ip_set_hash_netnet.c: In
function ?hash_netnet4_kadt?:
/usr/src/ipset/kernel/net/netfilter/ipset/ip_set_hash_netnet.c:141:
error: unknown field ?cidr? specified in initializer

Reported-by: Husnu Demir <hdemir@metu.edu.tr>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>