Add dm to DM_INCLUDES to fix Google3 build.

It seems that #including a file in the same directory works without the directory being in INCLUDES, but https://codereview.chromium.org/1631053003 #included DMGpuSupport.h from a test.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1657703002

Review URL: https://codereview.chromium.org/1657703002
diff --git a/public.bzl b/public.bzl
index 720a28e..83bfc1a 100644
--- a/public.bzl
+++ b/public.bzl
@@ -430,6 +430,7 @@
 ################################################################################
 
 DM_INCLUDES = [
+    "dm",
     "gm",
     "src/codec",
     "src/effects",