fixed a compiling error in sam7x BSP

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2515 bbd45198-f89e-11dd-88c7-29a3b14d5316

bsp/sam7x/drivers/sam7x_emac.c

@@ -5,6 +5,7 @@
 #include "sam7x_emac.h"
 #include "AT91SAM7X256.h"
 #include "lwipopts.h"
+#include "lwip/mem.h"
 
 #define MAX_ADDR_LEN 6