GN: Fix incorrect include_dir for video_coding on iOS

webrtc/modules/video_coding/BUILD.gn

Index: webrtc/modules/video_coding/BUILD.gn
diff --git a/webrtc/modules/video_coding/BUILD.gn b/webrtc/modules/video_coding/BUILD.gn
index d9b392d77ac685b120a51b0322bf585415b179c0..21539a2a8fad640f54d30ae325a703bc32677a58 100644
--- a/webrtc/modules/video_coding/BUILD.gn
+++ b/webrtc/modules/video_coding/BUILD.gn
@@ -218,7 +218,7 @@ if (is_ios) {
       ]
     } else {
       # Need to add a directory normally exported by libyuv.
-      include_dirs += [ "$rtc_libyuv_dir/include" ]
+      include_dirs = [ "$rtc_libyuv_dir/include" ]
     }
   }
 }