Add missing dependencies on Str and Unix

[dra27]

Provides compatibility to silence the alert proposed in ocaml/ocaml#11198. The change is compatible with previous OCaml releases, but it's obviously not worth merging this unless/until the OCaml PR is accepted.

[pmetzger]

Generally speaking, I think calendar isn't well maintained at the moment. I have no real objections, but IIRC which branch releases are actually cut from is a mess and that might need to be cleaned up to do another release.