parse-options: adjust `parse_opt_unknown_cb()`s declared return type

parse-options: adjust `parse_opt_unknown_cb()`s declared return type

In f41179f16ba2 (parse-options: avoid magic return codes, 2019-01-27),
the signature of the low-level parse-opt callback function was changed
to return an `enum`.

And while the implementations were changed, one declaration was left
unchanged, still claiming to return `int`.

This can potentially lead to problems, as compilers are free to choose
any integral type for an `enum` as long as it can represent all declared
values.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>