Expose server/data package in plugin API

In Ie4d6469eb1f server/data package was split from giant server target,
but the new package was missed to be exposed in the plugin API. As the
consequence the plugins cannot be built any more.

Change-Id: I6baa654644b7c52b2e13d518a036342c96dc9109

plugins/BUILD

@@ -49,6 +49,7 @@ EXPORTS = [
     "//java/com/google/gerrit/server/audit",
     "//java/com/google/gerrit/server/cache/mem",
     "//java/com/google/gerrit/server/cache/serialize",
+    "//java/com/google/gerrit/server/data",
     "//java/com/google/gerrit/server/logging",
     "//java/com/google/gerrit/server/schema",
     "//java/com/google/gerrit/server/util/time",