gsm_utils: Add osmo_dump_gsmtime

Used by osmo-bts, moved from osmo-bts l1sap.c:dump_gsmtime.

Change-Id: Ib5452e2c20f53006c0f6d197fb055728947125d8
diff --git a/include/osmocom/gsm/gsm_utils.h b/include/osmocom/gsm/gsm_utils.h
index 998f681..a879d33 100644
--- a/include/osmocom/gsm/gsm_utils.h
+++ b/include/osmocom/gsm/gsm_utils.h
@@ -168,6 +168,9 @@
 /* Convert from GSM time to frame number */
 uint32_t gsm_gsmtime2fn(struct gsm_time *time);
 
+/* Returns static buffer with string representation of a GSM Time */
+char *osmo_dump_gsmtime(const struct gsm_time *tm);
+
 /* GSM TS 03.03 Chapter 2.6 */
 enum gprs_tlli_type {
 	TLLI_LOCAL,