Add visibility from //plugins

Otherwise including the plugin in core Gerrit's CUSTOM_PLUGINS causes
the release build to fail with:

  Target '//plugins/gitiles:gitiles.jar' is not visible from target '//plugins:core'

Change-Id: Idb8e267376ce92a7b0c911561cfbe5fe431c7d6b
diff --git a/BUILD b/BUILD
index b5c7a38..db4ef4a 100644
--- a/BUILD
+++ b/BUILD
@@ -7,6 +7,7 @@
         "@gitiles_servlet//jar",
     ],
     outs = ["gitiles.jar"],
+    visibility = ["//plugins:__subpackages__"],
     cmd = " && ".join([
         "ROOT=$$PWD",
         "TMP=$$(mktemp -d || mktemp -d -t bazel-tmp)",