perfetto-ui: Copying directories is right out

We shouldn't copy directories in the build.

Change-Id: I1b967e2e257d67117400b60259a68034a30a3424
diff --git a/ui/BUILD.gn b/ui/BUILD.gn
index 7ebca4c..104b6e8 100644
--- a/ui/BUILD.gn
+++ b/ui/BUILD.gn
@@ -287,10 +287,15 @@
 
 copy("assets_dist") {
   sources = [
-    "src/assets",
+    "src/assets/flamegraph.svg",
+    "src/assets/logo-3d.png",
+    "src/assets/logo.png",
+    "src/assets/perfetto.scss",
+    "src/assets/sidebar.scss",
+    "src/assets/topbar.scss",
   ]
   outputs = [
-    "$ui_dir/assets/",
+    "$ui_dir/assets/{{source_file_part}}",
   ]
 }