ocaml-xen-lowlevel-libs: 'make build' rather than 'make'

Something is wrong with the docs build, but we're not packaging
these anyway.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #42 from djs55/io-page

ocaml-xen-lowlevel-libs: 'make build' rather than 'make'