Fix various compiler warnings in hnb-gw code

Since we finally started to use -Wall, the related warnings became
visible.

Change-Id: I516700eab2aa7c3412dd62775c4960aed9d4b682
diff --git a/src/context_map.c b/src/context_map.c
index 0960cb9..b90f555 100644
--- a/src/context_map.c
+++ b/src/context_map.c
@@ -169,4 +169,6 @@
 	context_map_tmr.cb = context_map_tmr_cb;
 	context_map_tmr.data = gw;
 	osmo_timer_schedule(&context_map_tmr, EXPIRY_TIMER_SECS, 0);
+
+	return 0;
 }