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/stdlib/lib_aligned_alloc.c

@@ -30,6 +30,7 @@
  * Public Functions
  ****************************************************************************/
 
+#undef aligned_alloc /* See mm/README.txt */
 FAR void *aligned_alloc(size_t align, size_t size)
 {
   return lib_memalign(align, size);