Include <stdlib.h> for malloc() on all platforms.

malloc.h is a nice idea, but there's a long list of platforms
where it isn't available. Considering it's just this file,
it's not worth trying to maintain a list of platform defines
to conditionalize malloc.h.

Addresses the first part of issue #227.
Merge pull request #228 from rillian/issue-227

Include <stdlib.h> for malloc() on all platforms.