cta: include stddef.h for size_t

Currently the CI on the main branch is failing:

2023/01/10 14:06:54 parse error: /builds/emersion/libdisplay-info/include/libdisplay-info/cta.h:544:9: unexpected identifier, expected ';'

I think the problem is that size_t is not defined.

It's a bit ugly that this failure only showed up on main so I also change the CI to always build the docs and only publish them on emersion/main.