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

This is required to get things linking in clang/win. Medium-term, we should
clean libyuv up to not require _posix files on non-posix.

Reviewed at https://webrtc-codereview.appspot.com/52069004/

BUG=496928

Review URL: https://webrtc-codereview.appspot.com/54689004
flykickbird/libyuv,flykickbird/libyuv,flykickbird/libyuv