fsck.h: use "enum object_type" instead of "int"

fsck.h: use "enum object_type" instead of "int"

Change the fsck_walk_func to use an "enum object_type" instead of an
"int" type. The types are compatible, and ever since this was added in
355885d5315 (add generic, type aware object chain walker, 2008-02-25)
we've used entries from object_type (OBJ_BLOB etc.).

So this doesn't really change anything as far as the generated code is
concerned, it just gives the compiler more information and makes this
easier to read.

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