Make x509 a regular dependency

[Leonidas-from-XIV]

x509 is required as a regular dependency, not doc-only, as

$ opam switch install . --deps-only --with-test
$ dune runtest

yields an error message like this:

File "dune", line 17, characters 28-32:
17 |  (libraries castore cstruct x509))
                                 ^^^^
Error: Library "x509" not found.
-> required by _build/default/decode_test.exe
-> required by alias runtest in dune:15
File "dune", line 2, characters 12-16:     
2 |  (libraries x509 castore))
                ^^^^
Error: Library "x509" not found.
-> required by _build/default/mdx_gen.bc
-> required by _build/default/.mdx/CHANGES.md.corrected
-> required by alias runtest in dune:1

Installing x509 manually using opam install x509 fixes this.

[leostera]

Thanks @Leonidas-from-XIV for the PR! ✨