net: include missing headers in net/net_namespace.h

Include linux/idr.h and linux/skbuff.h since they are required by objects that
are declared in the net structure.

 struct net {
	...
	struct idr		netns_ids;
	...
	struct sk_buff_head	wext_nlevents;
	...

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>