Have ocamlrep_ocamlpool depend on ocamlpool

Have ocamlrep_ocamlpool depend on ocamlpool

Summary:
For some reason, we never declared a dependency on the `ocamlpool` C library from the Rust bindings library `ocamlrep_ocamlpool`, instead requiring every binary rule which links to `ocamlrep_ocamlpool` to manually declare a dependency upon `ocamlpool`. I don't see any reason why this needs to be the case.

Also, do not use `ocamlrep_caml_builtins` in `oxidized_by_ref`, since pure-Rust binaries should be able to link to `oxidized_by_ref` without stubbing out the OCaml runtime. I'm not sure why they have been able to do this in the past, but we encounter linker errors in such binaries after adding the dependency between `ocamlrep_ocamlpool` and `ocamlpool`.

Reviewed By: shayne-fletcher

Differential Revision: D41417103

fbshipit-source-id: 19201bc1c8db30eedb2cf790009d077826a61d6b