main: use osmo_fsm_set_dealloc_ctx(OTC_SELECT)

Prepare to use osmo_mgcp_ep_alloc in a future patch, which requires
using osmo_fsm_set_dealloc_ctx(OTC_SELECT) for deferred FSM deallocation
(see the warning in src/libosmo-mgcp-client/mgcp_client_endpoint_fsm.c).

Related: SYS#5560
Change-Id: I97abf88fa64cc1a4906f356c57912593d4366a37

src/osmo-bsc-nat/main.c

@@ -184,6 +184,7 @@ int main(int argc, char **argv)
 
 	talloc_enable_null_tracking();
 	tall_bsc_nat_ctx = talloc_named_const(NULL, 0, "bsc_nat");
+	osmo_fsm_set_dealloc_ctx(OTC_SELECT);
 
 	rc = osmo_init_logging2(tall_bsc_nat_ctx, &bsc_nat_log_info);
 	if (rc < 0)