SDL: Add alsaLib to propagatedNativeBuildInputs.

This is needed because the pkgconfig file contains linker flags for
alsa-lib. And we had it propagated before already.

Should fix build of quite a lot of SDL dependencies, such as SDL_image:

https://hydra.nixos.org/build/10558332

Signed-off-by: aszlig <aszlig@redmoonstudios.org>