add #undef for some libc function

since it's useful to redirect these functions to others
sometime(e.g. validate the memory before write).

Change-Id: I6253a9231af8809e8362f4bc5a1bd67fb094c3b0

libs/libc/string/lib_strdup.c

@@ -32,6 +32,7 @@
  * Public Functions
  ****************************************************************************/
 
+#undef strdup /* See mm/README.txt */
 FAR char *strdup(FAR const char *s)
 {
   FAR char *news = NULL;