Use proper modifiers for size_t printing.

Eliminates build warnings.

src/stlink-common.c

@@ -1455,7 +1455,7 @@ int stlink_write_flash(stlink_t *sl, stm32_addr_t addr, uint8_t* base, unsigned
 		    fprintf(stdout, "\r");
 
 		if ((off % sl->flash_pgsz) > (sl->flash_pgsz -5)) {
-		    fprintf(stdout, "\r%3u/%u pages written", 
+		    fprintf(stdout, "\r%3zd/%3zd pages written",
 			    off/sl->flash_pgsz, len/sl->flash_pgsz);
 		    fflush(stdout);
 		}