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

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.