Fix constants.mojom-forward.h include

This was causing flaky compile failures on Linux ChromiumOS Full bots.

Example failure: https://ci.chromium.org/p/chromium/builders/ci/Linux%20ChromiumOS%20Full/139037

TBR=xiaohuic@chromium.org

Change-Id: I60b94045b5d55ff9c627351e25aa5bf25c199ead
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1732745Reviewed-by: Chris Sharp <csharp@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683679}

chromeos/services/assistant/BUILD.gn

@@ -137,6 +137,7 @@ source_set("tests") {
   testonly = true
   deps = [
     ":lib",
+    "//ash/public/mojom",
     "//base",
     "//base/test:test_support",
     "//chromeos/audio",

chromeos/services/assistant/service_unittest.cc

@@ -9,7 +9,7 @@
 #include <vector>
 
 #include "ash/public/cpp/voice_interaction_controller.h"
-#include "ash/public/interfaces/constants.mojom-forward.h"
+#include "ash/public/mojom/constants.mojom-forward.h"
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/macros.h"