audio_processing: Added back ATTRIBUTE_UNUSED lost in r7877
audio_processing: Added back ATTRIBUTE_UNUSED lost in r7877

BUG=N/A
TESTED=Now it builds with aec_debug_dump=1 on Mac
TBR=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/35629004

git-svn-id: 03ae4fbe531b1eefc9d815f31e49022782c42458@7986 4adac7df-926f-26a2-2b94-8c16560cd09d

svn2github/webrtc-Revision-8758,svn2github/webrtc-Revision-8758,svn2github/webrtc-Revision-8758,svn2github/webrtc-Revision-8758,svn2github/webrtc-Revision-8758,svn2github/webrtc-Revision-8758