mallinfo is meant to be API compatible with Linux,
where it's provided by malloc.h.
Make stdlib.h include malloc.h for now. It can be removed
once all users are updated to include malloc.h instead of
stdlib.h.
I have some app code which uses mallinfo(). I want to share
it between platforms. This commit allows me to reduce
platform-ifdefs.
(I think the API actually originated with System V. I don't
remember how it was there though. Anyway, I guess the
compatibility with Linux is more important than System V
these days.)
