Add function to get osmo_earfcn bit size

This function returns the number of bits necessary to pack osmo_earfcn
into Repeated E-UTRAN Neighbour Cells IE (3GPP TS 44.018 Table
10.5.2.33b.1).
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index a3e4e14..6886a6c 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -31,6 +31,7 @@
 
 osmo_earfcn_add;
 osmo_earfcn_del;
+osmo_earfcn_bit_size;
 osmo_earfcn_init;
 
 gprs_cipher_gen_input_i;