Fix compilation on FreeBSD

io-page-1.1.0 and git master fail with:
In file included from lib/stub_alloc_pages.c:37:0:
/usr/include/malloc.h:3:2: error: #error "<malloc.h> has been replaced by <stdlib.h>"

On FreeBSD we better not include malloc.h then.

This is tested on FreeBSD-CURRENT from 1st January 2014, ocaml 4.01.00
on amd64 using FreeBSD clang version 3.3 (tags/RELEASE_33/final 183502) 20130610

fixes issue #9 reported yesterday by myself