add gsm0808_cell_id_list_add() to combine two cell identifier lists

This will be used by the upcoming neighbor_ident API in osmo-bsc, where the vty
interface allows composing neihbor BSS cell identifier lists, and we want to
allow adding individual items from individual user commands.

It will also be useful to accumulate cell identifiers in case a subscriber sees
multiple alternative cells from a neighboring BSS, and we want to pass these on
to the MSC in a Handover Required.

Related: OS#2283 (inter-BSC HO, BSC side)
Change-Id: I5781f5fa5339c92ab2e2620489b002829d206925
diff --git a/include/osmocom/gsm/gsm0808_utils.h b/include/osmocom/gsm/gsm0808_utils.h
index 363fc39..34eae43 100644
--- a/include/osmocom/gsm/gsm0808_utils.h
+++ b/include/osmocom/gsm/gsm0808_utils.h
@@ -77,6 +77,7 @@
 int gsm0808_dec_cell_id_list(struct gsm0808_cell_id_list *cil,
 			     const uint8_t *elem, uint8_t len)
 			     OSMO_DEPRECATED("use gsm0808_dec_cell_id_list2 instead");
+int gsm0808_cell_id_list_add(struct gsm0808_cell_id_list2 *dst, const struct gsm0808_cell_id_list2 *src);
 int gsm0808_chan_type_to_speech_codec(uint8_t perm_spch);
 int gsm0808_speech_codec_from_chan_type(struct gsm0808_speech_codec *sc,
 					uint8_t perm_spch);