增加条件编译，防止RT_USING_DFS没有定义时 _sys_flen()出现编译警告：stat变量定义但没有被调用 的问题

Author: mysterywolf

components/libc/compilers/armlibc/stubs.c

@@ -9,6 +9,8 @@
  * 2013-11-24     aozima       fixed _sys_read()/_sys_write() issues.
  * 2014-08-03     bernard      If using msh, use system() implementation
  *                             in msh.
+ * 2020-08-05     Meco Man     fixed _sys_flen() compiling-warning when 
+ *                             RT_USING_DFS is not defined
  */
 
 #include <string.h>
@@ -265,7 +267,9 @@ RT_WEAK void _sys_exit(int return_code)
  */
 long _sys_flen(FILEHANDLE fh)
 {
+#ifdef RT_USING_DFS
     struct stat stat;
+#endif
 
     if (fh < STDERR)
         return -1;