Exclude some source files from the libk

[willdurand]

We use __is_libk to control what goes where (i.e. libc only, libk only or both) but now we have source files that contain code for libc only and gcc warns us:

warning: ISO C forbids an empty translation unit [-Wpedantic]

We should exclude source files (.c) we know shouldn't be in the libk given that libk should be a subset of libc.

[willdurand]

I don't know if that is still the case but the new(-ish) build system should allow us to only select the source files we want for libc and libk in a distinct manner.