CONTRIB: halog: fix issue with array of type char

I just noticed this in the windows build after moving the file to dev/:

   In file included from include/import/ist.h:32,
                   from include/haproxy/connection-t.h:32,
                   from dev/flags/flags.c:5:
  dev/flags/flags.c: In function `main':
  dev/flags/flags.c:442:20: error: array subscript has type `char' [-Werror=char-subscripts]
    442 |           (isalnum(*err) && toupper(*err) != 'U' && toupper(*err) != 'L'))
        |                    ^~~~
    LD      haproxy
  cc1: all warnings being treated as errors
  make: *** [Makefile:932: dev/flags/flags.o] Error 1
  make: *** Waiting for unfinished jobs....
  Error: Process completed with exit code 2.

Let's just cast it to uchar as is done everywhere else.

(cherry picked from commit b00c00e82ccffe61cb6c9de78c0a6dd6a6a40207)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/contrib/debug/flags.c b/contrib/debug/flags.c
index a596b1a..89d45e7 100644
--- a/contrib/debug/flags.c
+++ b/contrib/debug/flags.c
@@ -441,7 +441,7 @@
 			/* stop at the end of the number and trim any C suffix like "UL" */
 			err = value;
 			while (*err == '-' || *err == '+' ||
-			       (isalnum(*err) && toupper(*err) != 'U' && toupper(*err) != 'L'))
+			       (isalnum((unsigned char)*err) && toupper((unsigned char)*err) != 'U' && toupper((unsigned char)*err) != 'L'))
 				err++;
 			if (err)
 				*err = 0;