Bug 20273: Use compat routines for autocomplete in auth_finder.pl

Bug 20273: Use compat routines for autocomplete in auth_finder.pl

Test plan:
  - Context: Koha working with elasticsearch,
  - Apply BZ 19582 (that make auth_finder.pl works again),
  - apply this patch,
  - go to cgi-bin/koha/authorities/auth_finder.pl,
  - type at least 3 chararcters in a search field and
    check that the autocomplete returns results

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>