BUILD: cli: fix build on Windows due to isalnum() implemented as a macro

Commit 986798718 ("DEBUG: cli: add "debug dev task" to show/wake/expire/kill
tasks and tasklets") broke the build on windows due to this:

  src/debug.c:940:95: error: array subscript has type char [-Werror=char-subscripts]
    940 |  caller && may_access(caller) && may_access(caller->func) && isalnum(*caller->func) ? caller->func : "0",
        |                                                                      ^~~~~~~~~~~~~

It's classical on platforms which implement ctype.h as macros instead of
functions, let's cast it as uchar. No backport is needed.
diff --git a/src/debug.c b/src/debug.c
index b196fec..a68c45d 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -937,7 +937,7 @@
 		memprintf(&msg, "%s%p: %s state=%#x tid=%d process=%s ctx=%p calls=%d last=%s:%d intl=%d",
 			  msg ? msg : "", t, (t->state & TASK_F_TASKLET) ? "tasklet" : "task",
 			  t->state, t->tid, trash.area, t->context, t->calls,
-			  caller && may_access(caller) && may_access(caller->func) && isalnum(*caller->func) ? caller->func : "0",
+			  caller && may_access(caller) && may_access(caller->func) && isalnum((uchar)*caller->func) ? caller->func : "0",
 			  caller ? t->caller->line : 0,
 			  (t->state & TASK_F_TASKLET) ? LIST_INLIST(&((const struct tasklet *)t)->list) : 0);