netns: remove duplicated include from net_namespace.c

Remove duplicated include.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -16,7 +16,6 @@
 #include <linux/export.h>
 #include <linux/user_namespace.h>
 #include <linux/net_namespace.h>
-#include <linux/rtnetlink.h>
 #include <net/sock.h>
 #include <net/netlink.h>
 #include <net/net_namespace.h>