Update to Dune 3 `ctypes` stanza

[MisterDA]

Dune 3 has native support for ctypes. It now shows warnings when building ocaml-posix packages. I think it would be nicer to switch to the new stanza, but it's not clear to me how to proceed, seeing that we have C code being generated by hand. https://dune.readthedocs.io/en/stable/foreign-code.html#stub-generation-with-dune-ctypes

File "posix-types/src/generator/dune", line 19, characters 3-17:
19 |    %{lib:ctypes:}
        ^^^^^^^^^^^^^^
Error: The form %{lib:<libname>:} is no longer supported since version 3.0 of
the Dune language.
Hint: Did you know that Dune 3.0 supports ctypes natively? See the manual for
more details.