Cleanup: Forward declare AudioFrame type in voiceprocess.h

No need to include this header since the API is just taking a pointer to
it.

BUG=1092
TEST=./webrtc/build/gyp_webrtc && ninja -C out/Debug
R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44059004

Patch from Thiago Farina <tfarina@chromium.org>.

Cr-Commit-Position: refs/heads/master@{#8928}
2 files changed