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/gtphub.c b/openbsc/src/gprs/gtphub.c
index a1aaed2..211018b 100644
--- a/openbsc/src/gprs/gtphub.c
+++ b/openbsc/src/gprs/gtphub.c
@@ -2438,9 +2438,7 @@
 
 static void gtphub_gc_start(struct gtphub *hub)
 {
-	hub->gc_timer.cb = gtphub_gc_cb;
-	hub->gc_timer.data = hub;
-
+	osmo_timer_setup(&hub->gc_timer, gtphub_gc_cb, hub);
 	osmo_timer_schedule(&hub->gc_timer, GTPH_GC_TICK_SECONDS, 0);
 }