add gsm0808_cell_id_to_list()

The idea is to be able to add a gsm0808_cell_id to a gsm0808_cell_id_list2:
first convert it to a list, then re-use gsm0808_cell_id_list_add(). It will be
used by osmo-bsc to manage neighbor-BSS cell identifiers from VTY.

Change-Id: Ibf746ac60b1b1e920baf494b396658a5ceabd788
diff --git a/include/osmocom/gsm/gsm0808_utils.h b/include/osmocom/gsm/gsm0808_utils.h
index 24c139a..6555734 100644
--- a/include/osmocom/gsm/gsm0808_utils.h
+++ b/include/osmocom/gsm/gsm0808_utils.h
@@ -98,6 +98,7 @@
 			     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);
+void gsm0808_cell_id_to_list(struct gsm0808_cell_id_list2 *dst, const struct gsm0808_cell_id *src);
 uint8_t gsm0808_enc_cell_id(struct msgb *msg, const struct gsm0808_cell_id *ci);
 int gsm0808_dec_cell_id(struct gsm0808_cell_id *ci, const uint8_t *elem, uint8_t len);
 int gsm0808_chan_type_to_speech_codec(uint8_t perm_spch);