ggsn: Start gtp retrans timer during startup

This timer was added in osmo-ggsn.git
dda21ed7d4,
but it was never initially started since it was introducing, and as a
result retransmissions never being triggered.

Also as a consequence, gtp_retrans is never called. That function is
responsible from triggering retransmissions and to free old responses
waiting in the resp queue (to check for duplicates). Since it's never
called, the retransmit resp queue will grow over time.

Fixes: dda21ed7d4
Fixes: OS#3997
Change-Id: Ie4adc52829446539fbbb5e9e0cf75a04f91c7eea

ggsn/ggsn.c

@@ -1022,6 +1022,7 @@ int ggsn_start(struct ggsn_ctx *ggsn)
 
 	/* Start GTP re-transmission timer */
 	osmo_timer_setup(&ggsn->gtp_timer, ggsn_gtp_tmr_cb, ggsn);
+	ggsn_gtp_tmr_start(ggsn);
 
 	gtp_set_cb_data_ind(ggsn->gsn, encaps_tun);
 	gtp_set_cb_delete_context(ggsn->gsn, delete_context);