Fix compilation on FreeBSD by not including `<malloc.h>`

It looks like `malloc.h` is unnecessary on Linux, so just removing
this include lets the stubs compile on all our platforms.

Closes mirage/io-page#10

Acked-by: Hannes Mehnert
Merge pull request #11 from avsm/fix-freebsd-compile

Fix compilation on FreeBSD by not including `<malloc.h>`