non-iu-build: guard vty libosmo-sigtran function calls.

The function calls osmo_ss7_vty_go_parent() and gtphub_vty_is_config_node()
are not guarded by an ifdef. The functions are only needed for IU
support where libosmo-sigtran is used.

Guard the function calls with a BUILD_IU

Change-Id: I1c609ef0c344ce825c313335c7e084bf97159262