BUILD: ssl: only pass unsigned chars to isspace()

BUILD: ssl: only pass unsigned chars to isspace()

A build failure on cygwin was reported on github actions here:

  https://github.com/haproxy/haproxy/runs/466507874

It's caused by a signed char being passed to isspace(), and this one
being implemented as a macro instead of a function as the man page
suggests. It's the same issue that regularly pops up on Solaris. This
comes from commit 98263291cc3 which was merged in 1.8-dev1. A backport
is possible though not incredibly useful.

(cherry picked from commit ded15b75640d8f96929beefa3357cafbea25171e)
Signed-off-by: Willy Tarreau <w@1wt.eu>