builtin/maintenance: fix typedef for function pointers

builtin/maintenance: fix typedef for function pointers

The typedefs for `maintenance_task_fn` and `maintenance_auto_fn` are
somewhat confusingly not true function pointers. As such, any user of
those typedefs needs to manually add the pointer to make use of them.

Fix this by making these true function pointers.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>