python: add "public" visibility for Python library

Change-Id: I24f7bc83d77fa202ba6779fb602f374267ef6f25
Bug: 180653308
diff --git a/BUILD b/BUILD
index 7ca975e..422324e 100644
--- a/BUILD
+++ b/BUILD
@@ -3764,4 +3764,5 @@
         ":protobuf_descriptor_pb2_noop",
         ":pyglib_noop",
     ]
+    visibility = PERFETTO_CONFIG.public_visibility,
 )