Fix dialyzer warning

Author: prefiks

include/mod_private.hrl

@@ -19,6 +19,6 @@
 %%%----------------------------------------------------------------------
 
 -record(private_storage,
-        {usns = {<<"">>, <<"">>, <<"">>} :: {binary(), binary(), binary() |
+        {usns = {<<"">>, <<"">>, <<"">>} :: {binary() | '$1' | '_', binary(), binary() |
                                              '$1' | '_'},
          xml = #xmlel{} :: xmlel() | '_' | '$1'}).