GN: Fix incorrect include_dir for video_coding on iOS

When rtc_build_libyuv=false an incorrect code path
is surfaced in GN.

BUG=webrtc:6412
NOTRY=True
TESTED=gn gen out/foo --args='rtc_build_libyuv=false target_os="ios"'

Review-Url: https://codereview.webrtc.org/2375603002
Cr-Commit-Position: refs/heads/master@{#14392}

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" ]
     }
   }
 }