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.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 22985d5..3943d18 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -4591,7 +4591,7 @@
 				/* end of string, end of loop */
 				*line = 0;
 				break;
-			} else if (isspace(*line)) {
+			} else if (isspace((unsigned char)*line)) {
 				newarg = 1;
 				*line = 0;
 			} else if (*line == '[') {