Update dfs_elm.c

Author: mysterywolf

components/dfs/filesystems/elmfat/dfs_elm.c

@@ -945,7 +945,7 @@ DWORD get_fattime(void)
 {
     DWORD fat_time = 0;
 
-#ifdef RT_USING_RTC
+#ifdef RT_LIBC_USING_TIME
     time_t now;
     struct tm *p_tm;
     struct tm tm_now;