stanza: return const char* for non allocated strings

If interface function returns char* the result must be freed with
xmpp_free().

In case of const char* the result must not be changed by user. Also, the
result is valid only during stanza lifetime.

examples/active.c

@@ -24,7 +24,7 @@ int handle_reply(xmpp_conn_t * const conn,
 		 void * const userdata)
 {
     xmpp_stanza_t *query, *item;
-    char *type;
+    const char *type;
 
     type = xmpp_stanza_get_type(stanza);
     if (strcmp(type, "error") == 0)