CRAS: mix - Add missing forward declaration.
CRAS: mix - Add missing forward declaration.

BUG=none
TEST=builds

Change-Id: I52fade91c5a3fcbfcf7c84d95f17c3cd976ac6f5
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/37207
Reviewed-by: Hsinyu Chao <8b91762bdbe27d3121b311091d7ca5125e9a8df6@chromium.org>

drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd