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/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index c4b1b75..5a57429 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -177,6 +177,7 @@
 gsm0808_dec_cell_id_list;
 gsm0808_dec_cell_id_list2;
 gsm0808_cell_id_list_add;
+gsm0808_cell_id_to_list;
 gsm0808_enc_cell_id;
 gsm0808_dec_cell_id;
 gsm0808_cell_id_name;