Add an empty tools/BUILD file explicitely for Bazel
Add an empty tools/BUILD file explicitely for Bazel

To make sure that all plugins consistently have empty BUILD files
within their tools.

Change-Id: I14a7a61e61df1e735e64bc7a98f17a52b25fc147

GerritCodeReview/plugins_admin-console,GerritCodeReview/plugins_admin-console,GerritCodeReview/plugins_admin-console