amt: Remove duplicate include

Clean up the following includecheck warning:

./drivers/net/amt.c: net/protocol.h is included more than once.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/amt.c

@@ -12,7 +12,6 @@
 #include <linux/igmp.h>
 #include <linux/workqueue.h>
 #include <net/net_namespace.h>
-#include <net/protocol.h>
 #include <net/ip.h>
 #include <net/udp.h>
 #include <net/udp_tunnel.h>