Added printf format modifier for mem_size_t depending on heap size

src/include/lwip/mem.h

@@ -70,8 +70,10 @@ typedef size_t mem_size_t;
  */
 #if MEM_SIZE > 64000l
 typedef u32_t mem_size_t;
+#define MEM_SIZE_F U32_F
 #else
 typedef u16_t mem_size_t;
+#define MEM_SIZE_F U16_F
 #endif /* MEM_SIZE > 64000 */
 
 #if MEM_USE_POOLS