mm: Implement malloc_usable_size

defined here:
https://man7.org/linux/man-pages/man3/malloc_usable_size.3.html

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: I5988bb28375626188312bd0a0ba3fc12db66f644

include/malloc.h

@@ -25,6 +25,8 @@
  * Included Files
  ****************************************************************************/
 
+#include <stdlib.h>
+
 /****************************************************************************
  * Pre-processor Definitions
  ****************************************************************************/
@@ -55,6 +57,7 @@ extern "C"
 #endif
 
 struct mallinfo mallinfo(void);
+size_t malloc_usable_size(FAR void *ptr);
 
 #if defined(__cplusplus)
 }