[NETNS]: Should build with CONFIG_SYSCTL=n

Previous NETNS patches broke CONFIG_SYSCTL=n case

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index e06d7cf..61a28ff 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -8,7 +8,9 @@
 struct ipv4_devconf;
 
 struct netns_ipv4 {
+#ifdef CONFIG_SYSCTL
 	struct ctl_table_header	*forw_hdr;
+#endif
 	struct ipv4_devconf	*devconf_all;
 	struct ipv4_devconf	*devconf_dflt;
 };