Don't include malloc.h on *BSD

This problem is already reported upstream:
http://bugzilla.rawstudio.org/show_bug.cgi?id=622

Here's a copy of the bug description:
"RawSpeed/StdAfx.h includes <malloc.h>. The file exists on *BSD but
including it triggers an #error on FreeBSD and a #warning on OpenBSD and
DragonFlyBSD; nothing on NetBSD."

The patch just copies what was done for Mac OS X.