[bazel] Set hello_world example visibility to public
[bazel] Set hello_world example visibility to public

I was playing with 'bazel cquery //...' and was unable to run any
queries because of this error:

ERROR: $my_repo_path/release/BUILD:10:8: in pkg_tar_impl rule          \
//release:opentitan: target '//sw/device/examples/hello_world:package' \
is not visible from target '//release:opentitan'. Check the visibility \
declaration of the former target if you think the dependency is        \
legitimate

Signed-off-by: Dan McArdle <8fb240f11ec935676fcfd8aed04f42f9ee63efa8@google.com>

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