add codec to header search path

unblocks skia roller, as now some core .cpp files need access to SkCodec.h

Bug:
Change-Id: I0e1375bd167fffbf48b2b27632b213c2e009a868
Reviewed-on: https://pdfium-review.googlesource.com/16090
Commit-Queue: Mike Reed <reed@google.com>
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/skia/BUILD.gn b/skia/BUILD.gn
index 4e1bde1..7ca7c19 100644
--- a/skia/BUILD.gn
+++ b/skia/BUILD.gn
@@ -23,6 +23,7 @@
     "config",
     "ext",
     "//third_party/skia/include/c",
+    "//third_party/skia/include/codec",
     "//third_party/skia/include/config",
     "//third_party/skia/include/core",
     "//third_party/skia/include/effects",