mod_netconf: set tcp port of notification server

add configure option to set tcp port of websockets server
- refs #643
diff --git a/src/notification_module.h b/src/notification_module.h
index a005dc5..49a96cc 100644
--- a/src/notification_module.h
+++ b/src/notification_module.h
@@ -53,7 +53,9 @@
 typedef struct s {} server_rec;
 #endif
 
+#ifndef NOTIFICATION_SERVER_PORT
 #define NOTIFICATION_SERVER_PORT	8080
+#endif
 
 /**
  * \brief Notification module initialization