argv-array: fix bogus cast when freeing array

argv-array: fix bogus cast when freeing array

Since the array struct stores a "const char **" argv member
(for compatibility with most of our argv-taking functions),
we have to cast away the const-ness when freeing its
elements.

However, we used the wrong type when doing so.  It doesn't
make a difference since free() take a void pointer anyway,
but it can be slightly confusing to a reader.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>