char: cast ARRAY_SIZE() as signed to silent warning on empty array

chardev/char.c: In function 'chardev_name_foreach':
chardev/char.c:546:19: error: comparison of unsigned expression < 0 is always false [-Werror=type-limits]
     for (i = 0; i < ARRAY_SIZE(chardev_alias_table); i++) {
                   ^
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170530120919.8874-1-f4bug@amsat.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
