[NETNS]: Do no include NET related headers if CONFIG_NET is not set.

This fix broken compilation for 'allnoconfig'. This was introduced by
Introduced by commit 1218854a ("[NET]
NETNS: Omit seq_net_private->net without CONFIG_NET_NS.")

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>