builtin/fsck.c: don't conflate "int" and "enum" in callback

builtin/fsck.c: don't conflate "int" and "enum" in callback

Fix a warning on AIX's xlc compiler that's been emitted since my
a1aad71601a (fsck.h: use "enum object_type" instead of "int",
2021-03-28):

    "builtin/fsck.c", line 805.32: 1506-068 (W) Operation between
    types "int(*)(struct object*,enum object_type,void*,struct
    fsck_options*)" and "int(*)(struct object*,int,void*,struct
    fsck_options*)" is not allowed.

I.e. it complains about us assigning a function with a prototype "int"
where we're expecting "enum object_type".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>