audiofile: Use alsaLib for native Linux only.

Using ALSA for non-Linux systems really doesn't make much sense, so
using buildNativeInputs to ensure it won't be used for cross-builds.

Of course, if you're cross-building to Linux, the dependency won't be
used, but right now we don't have a good way to easily check whether the
cross target is Linux.

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