Add pubsub event callback

Also add generic message callback struct

src/xmpp/message.h

@@ -35,6 +35,10 @@
 #ifndef XMPP_MESSAGE_H
 #define XMPP_MESSAGE_H
 
+typedef int(*ProfMessageCallback)(xmpp_stanza_t *const stanza, void *const userdata);
+typedef void(*ProfMessageFreeCallback)(void *userdata);
+
 void message_handlers_init(void);
+void message_pubsub_event_handler_add(const char *const node, ProfMessageCallback func, ProfMessageFreeCallback free_func, void *userdata);
 
 #endif