vp8: Add missing call to SetUsageMessage().

Without it vp8_coder --help does not work.

BUG=None
TEST=ninja -C out/Debug && out/Debug/vp8_coder --help now shows the
usage message.

R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44649005

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

Cr-Commit-Position: refs/heads/master@{#8783}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8783 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed