ocaml: remove undefined behaviour in systemd_stubs.c

ocaml: remove undefined behaviour in systemd_stubs.c

Clang complains:

systemd_stubs.c:51:8: error: shifting a negative signed value is undefined [-Werror,-Wshift-negative-value]
        ret = Val_int(-1U);
              ^~~~~~~~~~~~

Since sd_notify_fd has a signature of unit -> unit, we simply change
the return value to Val_unit.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>