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

This fixes building with MSVC (and probably old GCC too) after
9636af88b892703ed13ce9216723c63029b6dd1a.

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.

videolan/dav1d