add osmo_bts_features_names: short BTS feature strings

This will be used by osmo-bts-omldummy to parse features strings from
the cmdline.

Note that osmo_bts_feature_name() already exists to return the longer
descriptive value_strings from osmo_bts_features_descs (_descs!).
Luckily that misses the plural 'features' in the name, so that I can
still add a properly named osmo_bts_features_name() function that only
returns the name, matching the common pattern used in osmocom code.

Related: SYS#4895
Change-Id: I699cd27512887d64d824be680303e70fff3677c1
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 0ea0678..7b2c18f 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -396,6 +396,7 @@
 gsm48_decode_lai2;
 osmo_bts_features_descs;
 osmo_bts_feature_name;
+osmo_bts_features_names;
 osmo_plmn_to_bcd;
 osmo_plmn_from_bcd;
 osmo_mcc_name;