Export pcm_mmap_avail

This is useful for timer based wakeups and for checking how much data
has been captured by a hotwording device.

Change-Id: I409201a5c9ce73abc807a9dd767beb3a32810f06
Signed-off-by: Dylan Reid <dgreid@chromium.org>
(cherry picked from commit 9074cfc4127cb841f0633b35895141f90da3f0d6)
2 files changed