gprs_mm_state_iu_fsm.c: fix: assign timer_cb to mm_state_iu_fsm

GCC warns us that 'pmm_state_fsm_timer_cb' is defined but not used.
This function was introduced in [1], but was not assigned to the FSM.

[1] I66c2ac0350cb074aefd9a22c5121acf723f239d3

Change-Id: Ib040befc87b2676aad2b8fe3671404fb3f5b030b

src/sgsn/gprs_mm_state_iu_fsm.c

@@ -140,6 +140,7 @@ struct osmo_fsm mm_state_iu_fsm = {
 	.states = mm_state_iu_fsm_states,
 	.num_states = ARRAY_SIZE(mm_state_iu_fsm_states),
 	.event_names = mm_state_iu_fsm_event_names,
+	.timer_cb = pmm_state_fsm_timer_cb,
 	.log_subsys = DMM,
 };