Fix gn check for //chromeos/services/libassistant:internal after r9…

…05012.

gn check is failing on https://ci.chromium.org/p/chrome/builders/ci/linux-chromeos-chrome
since https://crrev.com/c/3046838. //chrome/dbus/util was
made into a target, and libassistant needs to depend on it now.

Error is
```
ERROR at //chromeos/services/libassistant/util.cc:19:11: Can't include this header from here.
#include "chromeos/dbus/util/version_loader.h"
          ^----------------------------------
The target:
  //chromeos/services/libassistant:internal
is including a file from the target:
  //chromeos/dbus/util:util
```

Bug: 1225516, 1232828
Change-Id: Id2b89cc0bc7468fe4056eace9abb3676c668bdd6
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3052874
Commit-Queue: Trent Apted <tapted@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@google.com>
Owners-Override: Andrew Xu <andrewxu@google.com>
Owners-Override: François Degros <fdegros@chromium.org>
Reviewed-by: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/master@{#905108}

chromeos/services/libassistant/BUILD.gn

@@ -126,6 +126,7 @@ source_set("internal") {
     "//chromeos/assistant/internal/proto:assistant",
     "//chromeos/dbus",
     "//chromeos/dbus/power",
+    "//chromeos/dbus/util:util",
     "//chromeos/resources:resources_grit",
     "//chromeos/services/assistant/public/cpp",
     "//chromeos/services/assistant/public/proto",