[NETNS]: Compile NET /proc support only if CONFIG_NET is set.

This fix broken compilation for 'allnoconfig'. This was introduced by
Introduced by commit 1218854afa6f659be90b748cf1bc7badee954a35 ("[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>
diff --git a/fs/proc/proc_net.c b/fs/proc/proc_net.c
index 13cd783..7034fac 100644
--- a/fs/proc/proc_net.c
+++ b/fs/proc/proc_net.c
@@ -51,6 +51,7 @@
 }
 EXPORT_SYMBOL_GPL(seq_open_net);
 
+#ifdef CONFIG_NET
 int seq_release_net(struct inode *ino, struct file *f)
 {
 	struct seq_file *seq;
@@ -218,3 +219,4 @@
 
 	return register_pernet_subsys(&proc_net_ns_ops);
 }
+#endif /* CONFIG_NET */