BUILD: http_htx: fix ci compilation error with isdigit for Windows

The warning is encountered on platforms for which char type is signed by
default.

cf the following links
https://stackoverflow.com/questions/10186219/array-subscript-has-type-char

This must be backported up to 2.4.

(cherry picked from commit 164ae4ad55bdd36d22cba8568227895ef5ddc086)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed