Avoid leaking a dependency on stdatomic.h outside of the main library

This fixes building with MSVC (and probably old GCC too) after 9636af88.

That commit made src/cpu.h include src/internal.h. src/internal.h includes stdatomic.h and thus requires declaring the dependency on stdatomic_dependencies.

Instead of adding that dependency in all other build targets that include src/cpu.h (most tests and tools), avoid leaking the dependency in this particular header.