[bazel] Expose Perfetto favicon.png asset

This allows Bazel clients to embed the `favicon.png` asset from
Perfetto in their products. That way, if it changes, the Bazel
clients will get the updated asset automatically.

Change-Id: I3b4394c112c0fb6959e315c5f9b69e91fb1fff59

BUILD

diff --git a/BUILD b/BUILD
index 2a5369f..903c2db 100644
--- a/BUILD
+++ b/BUILD
@@ -5627,3 +5627,8 @@
     main = "tools/write_version_header.py",
     python_version = "PY3",
 )
+
+exports_files(
+    ["ui/src/assets/favicon.png"],
+    visibility = PERFETTO_CONFIG.public_visibility,
+)