dropmon: remove duplicated #include

Remove duplicated #include('s) in
  include/linux/net_dropmon.h

Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/net_dropmon.h

@@ -3,7 +3,6 @@
 
 #include <linux/types.h>
 #include <linux/netlink.h>
-#include <linux/types.h>
 
 struct net_dm_drop_point {
 	__u8 pc[8];