[lwIP] fix compiling warning in NAT.

Author: BernardXiong

components/net/lwip_nat/ipv4_nat.c

@@ -103,6 +103,7 @@
 #include "lwip/udp.h"
 #include "lwip/mem.h"
 #include "lwip/sys.h"
+#include "lwip/timers.h"
 #include "netif/etharp.h"
 
 #include <limits.h>