Fix bzl missing include dir.

No-Try: true
Change-Id: Ic9b22cf8859417b6f757124a12903e630af94feb
Reviewed-on: https://skia-review.googlesource.com/156003
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
diff --git a/public.bzl b/public.bzl
index 32b1371..ab33c75 100644
--- a/public.bzl
+++ b/public.bzl
@@ -399,6 +399,7 @@
     "include/codec",
     "include/config",
     "include/core",
+    "include/docs",
     "include/effects",
     "include/encode",
     "include/gpu",