Successful build of bechamel for Alpine 3.19 and OCaml 5.1

[shakthimaan]

The CI build failure for bechamel for Alpine 3.19 and OCaml 5.1 is at: https://ocaml.ci.dev/github/mirage/bechamel/commit/a54a0b47300ed4ae9b95d4b05af701012caa31ca

The above can be fixed with the following two changes:

1. Install linux-headers using apk before proceeding to install the dependencies.

   $ sudo apk add linux-headers

2. Add result package to depends section in bechamel.opam.

I was then able to successfully build and run the tests:

$ sudo docker pull ocaml/opam:alpine-3.19-ocaml-5.1 
$ sudo docker run -it ocaml/opam:alpine-3.19-ocaml-5.1 bash
$ git clone https://github.com/mirage/bechamel.git
$ cd bechamel
$ sudo apk add linux-headers
$ opam install . --deps-only --with-test
$ dune build
$ dune runtest
$ dune exec -- ./_build/default/examples/list.exe
$ dune exec -- ./_build/default/examples/fact.exe