utils: add function gsm_fn_as_gsmtime_str()

Convert a given frame number into a printable string that displays
the sub components of the frame number.

Change-Id: I8015d2ded3940b01b35df7b72fc35c70c25e9926
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index ad7e013..714cb25 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -272,6 +272,7 @@
 gsm_band_name;
 gsm_band_parse;
 gsm_fn2gsmtime;
+gsm_fn_as_gsmtime_str;
 gsm_get_octet_len;
 gsm_gsmtime2fn;
 osmo_dump_gsmtime;