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
1 file changed