This fix brings common_video/BUILD.gn on a par with other BUILD.gn files that use $rtc_libyuv_dir to include libyuv header files when building WebRTC. Doing so eliminates the failure in the building process when a non-default $rtc_libyuv_dir is used. Bug: webrtc:8399 Change-Id: I1bac0285f1869a334d116d0e1371aa10204137e5 Reviewed-on: https://webrtc-review.googlesource.com/11140 Reviewed-by: Kári Helgason <kthelgason@webrtc.org> Reviewed-by: Magnus Jedvert <magjed@webrtc.org> Commit-Queue: Magnus Jedvert <magjed@webrtc.org> Cr-Commit-Position: refs/heads/master@{#20315}