Bazel: Add mail package to plugin API

In Ie5b22908fe server package was split off the server package, but it
was missed to expose the extacted package to the plugin API.

Change-Id: I8b348512c1d2031648a93c80bf0bdc1f2e3e9d37
diff --git a/plugins/BUILD b/plugins/BUILD
index 39d4260..2b0c0d4 100644
--- a/plugins/BUILD
+++ b/plugins/BUILD
@@ -36,6 +36,7 @@
     "//java/com/google/gerrit/index/project",
     "//java/com/google/gerrit/index:query_exception",
     "//java/com/google/gerrit/lifecycle",
+    "//java/com/google/gerrit/mail",
     "//java/com/google/gerrit/metrics",
     "//java/com/google/gerrit/metrics/dropwizard",
     "//java/com/google/gerrit/reviewdb:server",