Make documentation bzl_library publicly visible (#527)

This allows anyone who depends on `rules_jvm_external` and imports
"something" from these rules into their own rules to be able to
generate stardoc of their own functions.

I've filed bazelbuild/stardoc#93 to track
this issue. Once it's fixed, we can close this.

BUILD

@@ -52,6 +52,12 @@ bzl_library(
  "//settings:stamp_manifest.bzl",
  "//third_party/bazel_json/lib:json_parser.bzl",
  ],
+ visibility = [
+ # This library is only visible to allow others who depend on
+ # `rules_jvm_external` to be able to document their code using
+ # stardoc.
+ "//visibility:public",
+ ],
 )
 
 genrule(