tools/ocaml/libs: Don't declare stubs as taking void

tools/ocaml/libs: Don't declare stubs as taking void

There is no such thing as an Ocaml function (C stub or otherwise) taking no
parameters.  In the absence of any other parameters, unit is still passed.

This doesn't explode with any ABI we care about, but would malfunction for an
ABI environment such as stdcall.

Fixes: c3afd398ba7f ("ocaml: Add XS bindings.")
Fixes: 8b7ce06a2d34 ("ocaml: Add XC bindings.")
Signed-off-by: Edwin Török <edwin.torok@cloud.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
(cherry picked from commit ff8b560be80b9211c303d74df7e4b3921d2bb8ca)