Add missing BUILD dependency

This caused linking issues on a debug build (I do not know why it
doesn't cause issues on a non-debug build).

[0/1/1] SOLINK ./lib_libassistant_service.so
FAILED: lib_libassistant_service.so lib_libassistant_service.so.TOC
python "../../build/toolchain/gcc_solink_wrapper.py" <snip>
ld.lld: error: undefined symbol: NewLibassistantEntrypoint
>>> referenced by libassistant_entrypoint_client.cc:40 <snip>

Bug: b/171748795
Test: compiled
Change-Id: Id302592f9bd1623656403da530c05899a22cfc7d
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636682
Auto-Submit: Jeroen Dhollander <jeroendh@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844960}

chromeos/services/libassistant/BUILD.gn

@@ -61,6 +61,7 @@ source_set("internal") {
     ":fake_input_device",
     "//build/util:webkit_version",
     "//chromeos/assistant/internal",
+    "//chromeos/assistant/internal:libassistant",
     "//chromeos/assistant/internal/proto/google3",
     "//chromeos/dbus",
     "//chromeos/services/assistant/public/cpp",