ike: Allow creation of internally used payloads

Since 42e0a317c6 ("ike: Only parse payloads valid for the current IKE
version") payload types are checked before creating objects.  This check
failed for internally used payload types (e.g. proposal substructures),
which have a type >= 256, i.e. outside the IKE payload type range.

src/libcharon/encoding/payloads/payload.c

@@ -268,7 +268,7 @@ payload_t *payload_create(payload_type_t type)
  */
 bool payload_is_known(payload_type_t type, u_int8_t maj_ver)
 {
-	if (type == PL_HEADER)
+	if (type >= PL_HEADER)
 	{
 		return TRUE;
 	}