vty: use install_lib_element() and install_lib_element_ve()

See https://lists.osmocom.org/pipermail/openbsc/2020-October/013278.html.

Change-Id: Ic541126ffd4975daf87199abfafb465e2055e44f
Related: SYS#4937
diff --git a/src/vty/tdef_vty.c b/src/vty/tdef_vty.c
index fe6d48b..0556d8c 100644
--- a/src/vty/tdef_vty.c
+++ b/src/vty/tdef_vty.c
@@ -379,8 +379,8 @@
 	cfg_timer_cmd.string = timer_command_string("timer", OSMO_TDEF_VTY_ARG_SET_OPTIONAL);
 	cfg_timer_cmd.doc = timer_doc_string("Configure or show timers\n", OSMO_TDEF_VTY_DOC_SET);
 
-	install_element_ve(&show_timer_cmd);
-	install_element(parent_cfg_node, &cfg_timer_cmd);
+	install_lib_element_ve(&show_timer_cmd);
+	install_lib_element(parent_cfg_node, &cfg_timer_cmd);
 }
 
 /*! Write the global osmo_tdef_group configuration to VTY, as previously passed to osmo_tdef_vty_groups_init().