[NET]: netns compilation speedup

This patch speedups compilation when net_namespace.h is changed.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/sock.h b/include/net/sock.h
index bcbf889..800e73a 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -57,7 +57,6 @@
 #include <asm/atomic.h>
 #include <net/dst.h>
 #include <net/checksum.h>
-#include <net/net_namespace.h>
 
 /*
  * This structure really needs to be cleaned up.
@@ -95,6 +94,7 @@
 
 struct sock;
 struct proto;
+struct net;
 
 /**
  *	struct sock_common - minimal network layer representation of sockets