Fix building with MSVC after recent commit
98b0c96d added an include of src/ref.h in src/fg_apply_tmpl.c. That template source file is included in tests/checkasm/filmgrain.c.
src/ref.h includes <stdatomic.h>. Including this file requires declaring a dependency on stdatomic_dependencies in meson, which provides the fallback implementation of stdatomic.h when building with MSVC.