common_audio: Made input signal const in WebRtcSplFilterMAFastQ12()

BUG=3353, 1133
TESTED=locally on Mac and trybots
R=henrik.lundin@webrtc.org, kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/37499004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@8037 4adac7df-926f-26a2-2b94-8c16560cd09d
3 files changed