Update cstruct to 1.4.0

Signed-off-by: David Scott <dave.scott@citrix.com>
ocaml-cstruct: fix RPM build

Signed-off-by: David Scott <dave.scott@citrix.com>
ocaml-cohttp: fix RPM build

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #564 from djs55/upgrade-cstruct

Update cstruct to 1.4.0