move ts_sublots() to gsm_data_shared.c, it will be used by osmo-bts

Change-Id: I8ba06d7dd6e0ceab3d8d18bb565354d6ed461f7e
diff --git a/openbsc/include/openbsc/gsm_data_shared.h b/openbsc/include/openbsc/gsm_data_shared.h
index 953bac6..ce2e9b7 100644
--- a/openbsc/include/openbsc/gsm_data_shared.h
+++ b/openbsc/include/openbsc/gsm_data_shared.h
@@ -853,4 +853,6 @@
 struct gsm_lchan *rsl_lchan_lookup(struct gsm_bts_trx *trx, uint8_t chan_nr,
 				   int *rc);
 
+uint8_t ts_subslots(struct gsm_bts_trx_ts *ts);
+
 #endif