Add url_launcher dependency to BUILD.gn. (#9886)

diff --git a/examples/flutter_gallery/BUILD.gn b/examples/flutter_gallery/BUILD.gn
index eff7bb8..815260a 100644
--- a/examples/flutter_gallery/BUILD.gn
+++ b/examples/flutter_gallery/BUILD.gn
@@ -13,6 +13,7 @@
 
   deps = [
     "//lib/flutter/packages/flutter",
+    "//third_party/dart-pkg/pub/url_launcher",
   ]
 
   # TODO(chinmaygarde): Assets must be packaged in the FLX.