Include the header <cmath>

The build breaks because there is no implementation of std::abs(double).

BUG=None
TBR=tkchin@webrtc.org

Review-Url: https://codereview.webrtc.org/2743063003
Cr-Original-Commit-Position: refs/heads/master@{#17176}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9a1604b0273924f1d86b4499c00db65a09d4afce
1 file changed