src: use osmo_timer_setup()

Use new function available in libosmocore to set up timers. Compile
tested only.

Change-Id: Ibcfd915688e97d370a888888a83a7c95cbe16819
diff --git a/openbsc/src/gprs/sgsn_libgtp.c b/openbsc/src/gprs/sgsn_libgtp.c
index c26abc9..001e611 100644
--- a/openbsc/src/gprs/sgsn_libgtp.c
+++ b/openbsc/src/gprs/sgsn_libgtp.c
@@ -845,8 +845,7 @@
 	}
 
 	/* Start GTP re-transmission timer */
-	sgi->gtp_timer.cb = sgsn_gtp_tmr_cb;
-	sgi->gtp_timer.data = sgi;
+	osmo_timer_setup(&sgi->gtp_timer, sgsn_gtp_tmr_cb, sgi);
 	sgsn_gtp_tmr_start(sgi);
 
 	/* Register callbackcs with libgtp */