netfilter: nf_tables: remove duplicated include from nf_tables_ipv4.c

Remove duplicated include.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/nf_tables_ipv4.c

@@ -16,7 +16,6 @@
 #include <net/netfilter/nf_tables.h>
 #include <net/net_namespace.h>
 #include <net/ip.h>
-#include <net/net_namespace.h>
 #include <net/netfilter/nf_tables_ipv4.h>
 
 static unsigned int nft_ipv4_output(const struct nf_hook_ops *ops,