Use osmo_clock_gettime from libosmocore

Change-Id: Ic638849c6687c376c4c0c36cc286d499a073d6ca
diff --git a/src/gprs/gprs_sgsn.c b/src/gprs/gprs_sgsn.c
index 9046157..d8bca85 100644
--- a/src/gprs/gprs_sgsn.c
+++ b/src/gprs/gprs_sgsn.c
@@ -936,7 +936,7 @@
 
 	int rc;
 
-	rc = clock_gettime(CLOCK_MONOTONIC, &now_tp);
+	rc = osmo_clock_gettime(CLOCK_MONOTONIC, &now_tp);
 	OSMO_ASSERT(rc >= 0);
 	now = now_tp.tv_sec;