builtin/pack-objects.c: do not open-code `MAX_PACK_OBJECT_HEADER`

builtin/pack-objects.c: do not open-code `MAX_PACK_OBJECT_HEADER`

The function `write_reused_pack_one()` defines an header to store the
OFS_DELTA header, but uses the constant "10" instead of
"MAX_PACK_OBJECT_HEADER" (as is done elsewhere in the same patch, circa
bb514de356c (pack-objects: improve partial packfile reuse, 2019-12-18)).

Declare the `ofs_header` field to be sized according to
`MAX_PACK_OBJECT_HEADER` (which is 10, as defined in "pack.h") instead
of the constant 10.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>