client messages CHANGE add function to free client-side NETCONF error-reply information

Only internal function moving process of freeing nc_err data into a
standalone function to be usable at multiple places.
diff --git a/src/messages_p.h b/src/messages_p.h
index c5a0a6f..a4b04b6 100644
--- a/src/messages_p.h
+++ b/src/messages_p.h
@@ -188,4 +188,6 @@
 
 void nc_server_rpc_free(struct nc_server_rpc *rpc, struct ly_ctx *ctx);
 
+void nc_client_err_clean(struct nc_err *err, struct ly_ctx *ctx);
+
 #endif /* NC_MESSAGES_P_H_ */