parseopt: restore const qualifier to parsed filename

parseopt: restore const qualifier to parsed filename

This was unintentionally dropped in ccfcaf399f (parseopt: values of
pathname type can be prefixed with :(optional), 2025-09-28). Notably,
continue dropping the const qualifier when free'ing value; see
4049b9cfc0 (fix const issues with some functions, 2007-10-16) or
83838d5c1b (cast variable in call to free() in builtin/diff.c and
submodule.c, 2011-11-06) for more details on why.

Suggested-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>