domains: c: add missing intersphinx object types

[utzig]

For intersphinx to be able to resolve references to C-Domain objects, it needs to have them declared in the obj_types dict. This is currently missing enum, enumerator, struct and union so those can't be referenced by an external project. This commit fixes the issue by adding them.

[utzig]

@jakobandersen This is one commit broken away from #8160. I am adding a project attached, with the current build already there in the html folder, but you can run them yourself if you prefer so. If you look at the obj_types html folder, everything is OK, but ext_proj which uses intersphinx to resolve the symbols some are broken (and fixed by this commit). I also found out :c:member: does not work and :c:macro can't have the () for it to work, but those are not my focus right now...
c_intersphinx_samples.tar.gz

[utzig]

@jakobandersen This is one commit broken away from #8160. I am adding a project attached, with the current build already there in the html folder, but you can run them yourself if you prefer so. If you look at the obj_types html folder, everything is OK, but ext_proj which uses intersphinx to resolve the symbols some are broken (and fixed by this commit). I also found out :c:member: does not work and :c:macro can't have the () for it to work, but those are not my focus right now...
c_intersphinx_samples.tar.gz

Ah, I also added struct and union because they were broken, although my initial commit was only concerned with enum and enumerator!

[jakobandersen]

Thanks!