Fix DM_INCLUDES after https://codereview.chromium.org/1688543002

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1719253002

Review URL: https://codereview.chromium.org/1719253002
diff --git a/public.bzl b/public.bzl
index 77caaf1..0fb0cea 100644
--- a/public.bzl
+++ b/public.bzl
@@ -443,6 +443,7 @@
     "gm",
     "src/codec",
     "src/effects",
+    "src/effects/gradients",
     "src/fonts",
     "src/pathops",
     "src/pipe/utils",