Add default visibility to Bazel's config_settings. (proxy-wasm#311)

Signed-off-by: Keith Smiley <[email protected]>

Author: keith

bazel/BUILD

@@ -1,5 +1,7 @@
 load("@bazel_skylib//lib:selects.bzl", "selects")
 
+package(default_visibility = ["//visibility:public"])
+
 config_setting(
     name = "engine_null",
     values = {"define": "engine=null"},