Bug 13544: Make it explicit that getauthtypes returns a hash ref

Bug 13544: Make it explicit that getauthtypes returns a hash ref

Prior to perl 5.12 keys can only operate on a hash.

Test plan:
With perl 5.10, access to admin/auth_subfields_structure.pl.
Without this patch, you get:
Type of arg 1 to keys must be hash (not subroutine entry) at
/home/koha/src/admin/auth_subfields_structure.pl line 102, near
"getauthtypes)"

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Confirmed problem and tested patch on a sandbox, signed off locally.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>