netfilter: nf_conntrack_tuple_common.h: fix #include

To allow usage of enum ip_conntrack_dir in include/net/netns/conntrack.h,
this patch encloses #include <linux/netfilter.h> in a #ifndef __KERNEL__
directive, so that compiler errors caused by unwanted inclusion of
include/linux/netfilter.h are avoided.
In addition, #include <linux/netfilter/nf_conntrack_common.h> line has
been added to resolve correctly CTINFO2DIR macro.

Signed-off-by: Davide Caratti <[e-mail]>
Acked-by: Mikko Rapeli <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/uapi/linux/netfilter/nf_conntrack_tuple_common.h

···
 #define _NF_CONNTRACK_TUPLE_COMMON_H
 
 #include <linux/types.h>
+#ifndef __KERNEL__
 #include <linux/netfilter.h>
+#endif
+#include <linux/netfilter/nf_conntrack_common.h> /* IP_CT_IS_REPLY */
 
 enum ip_conntrack_dir {
 	IP_CT_DIR_ORIGINAL,