Apply patch from Ambroz Bizjak:

I think that "#define SZT_F PRIuPTR" has a much better chance
of being right, making the assumption that uintptr_t is the same as
size_t.
Dirk: I think Ambroz is right.

src/include/lwip/arch.h

@@ -106,7 +106,7 @@ typedef uintptr_t mem_ptr_t;
 #define X32_F PRIx32
 #endif
 #ifndef SZT_F
-#define SZT_F PRIuMAX
+#define SZT_F PRIuPTR
 #endif
 #endif