fix ash_perftests error

Currently ash_perftests fails to build due to undefined symbol
errors of keyboard::mojom::KeyboardConfig::KeyboardConfig() in
ash/public/cpp/test/test_keyboard_controller_observer.cc

Bug: none
Test: autoninja ash_perftests
Change-Id: Ib6c8ecd34295269ee27a1f63eb62f2c7a582f006
Reviewed-on: https://chromium-review.googlesource.com/c/1352044Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611780}

ash/public/cpp/BUILD.gn

@@ -202,6 +202,7 @@ source_set("test_support") {
     "//services/service_manager/public/cpp",
     "//ui/aura",
     "//ui/gfx",
+    "//ui/keyboard:mojom",
     "//ui/views",
   ]
 }