library/GTPv2_Emulation: fix timer not being started in f_uecups_xceive()

This change fixes the following error:

  Dynamic test case error: None of the branches can be chosen in
                           the alt statement in file GTPv2_Emulation.ttcn
                           between lines 362 and 370.

which may happen when osmo-uecups-daemon fails to create a tunnel
due to insufficient permissions.

Change-Id: Icb229aed7fa9dedcf999837ec6ad249ab4fdebb7
Related: SYS#5602
diff --git a/library/GTPv2_Emulation.ttcn b/library/GTPv2_Emulation.ttcn
index 8074269..f07afcb 100644
--- a/library/GTPv2_Emulation.ttcn
+++ b/library/GTPv2_Emulation.ttcn
@@ -359,6 +359,7 @@
 	var UECUPS_RecvFrom mrf;
 
 	UECUPS.send(t_UECUPS_Send(g_uecups_conn_id, tx));
+	T.start;
 	alt {
 	[] UECUPS.receive(tr_UECUPS_RecvFrom_R(rx_t)) -> value mrf { }
 	[] UECUPS.receive(tr_SctpAssocChange) { repeat; }