The definition of zalloc is trivial, so let's just have it here instead
of loading libweston/zalloc.h.

Now xalloc.h does not depend on any libweston header, which makes me
feel slightly better. It's more clean.

Who knows, maybe one day libweston/zalloc.h will be removed.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>