netfilter: netns nf_conntrack: add netns boilerplate

One comment: #ifdefs around #include is necessary to overcome amazing compile
breakages in NOTRACK-in-netns patch (see below).

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index a8eb43c..708009b 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -16,6 +16,9 @@
 #include <net/netns/ipv6.h>
 #include <net/netns/dccp.h>
 #include <net/netns/x_tables.h>
+#if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
+#include <net/netns/conntrack.h>
+#endif
 
 struct proc_dir_entry;
 struct net_device;
@@ -67,6 +70,9 @@
 #endif
 #ifdef CONFIG_NETFILTER
 	struct netns_xt		xt;
+#if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
+	struct netns_ct		ct;
+#endif
 #endif
 	struct net_generic	*gen;
 };