Add Riku Voipio to AUTHORS.

Contributed this openmax patch:
crrev.com/4636f5bb744c0828ac853e1a513e375886fcd424

R=riku.voipio@linaro.org

Review URL: https://codereview.webrtc.org/1430973006 .

Cr-Commit-Position: refs/heads/master@{#10543}
diff --git a/AUTHORS b/AUTHORS
index 0d9ff5a..3ecc088 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -22,6 +22,7 @@
 Paul Kapustin <pkapustin@gmail.com>
 Rafael Lopez Diez <rafalopezdiez@gmail.com>
 Ralph Giles <giles@ghostscript.com>
+Riku Voipio <riku.voipio@linaro.org>
 Robert Nagy <robert.nagy@gmail.com>
 Ryan Yoakum <ryoakum@skobalt.com>
 Sarah Thompson <sarah@telergy.com>