Revert "libyuv: Include foo_posix.cc in the gn build, like it's done in the gyp build."
Revert "libyuv: Include foo_posix.cc in the gn build, like it's done in the gyp build."

BUG=496928
R=thakis@chromium.org

Review URL: https://webrtc-codereview.appspot.com/58399004
flykickbird/libyuv,flykickbird/libyuv,flykickbird/libyuv