Use $rtc_libyuv_dir in common_video/BUILD.gn, not hard-encoded "libyuv"

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}

common_video/BUILD.gn

@@ -11,7 +11,7 @@ import("../webrtc.gni")
 config("common_video_config") {
   include_dirs = [
     "include",
-    "libyuv/include",
+    "$rtc_libyuv_dir/include",
   ]
 }