Fix C stubs dependency specification in mirage-crypto-ec

[TheLortex]

When using mirage-crypto-ec with Mirage 4, I've encountered build failures as mirage_crypto.h couldn't be found when building mirage-crypto-ec's stubs. Indeed dune does not deduce from the -I../src/native flag that it has to track ../src/native as a dependency folder.

Instead the include_dirs option should be used: that's what this PR is about.

In the opam world this hasn't been a problem because the dune file in ec-freestanding correctly specifies the dependency.

For more information on the dune options for C compilation: https://dune.readthedocs.io/en/stable/concepts.html#foreign-stubs

[hannesm]

Thanks. Your change looks good, though I don't like the wording "fix" (neither in the commit message nor in the changes entry). from my perspective, it wasn't broken before - and if dune does not support -I .. (or gets confused by its usage), it should complain about its usage and recommend to use include_dirs.

if you could rephrase that (and mention why and what you changed in this commit), I'm happy to merge this.

[TheLortex]

Thank you for your review. Indeed nothing was "broken" before, but this is by luck, as the header files were grabbed when coincidentally building ec-freestanding. To me, we're "fixing" an issue that's upcoming (when the ec-freestanding build gets removed) -- but I've changed the changes entry and the commit message.

With regards to dune not supporting -I .., it's just that it doesn't parse C flags, it merely transfers them to the underlying C compiler, that's why the include_dirs field exists, and because it's not parsing the flags it cannot complain about the usage of the -I option. Additionally, the extra_deps field is available to track any dependency that would occur in the C build.

I hope that makes sense to you !

[hannesm]

thanks.