apparmor: avoid -Wempty-body warning

Building with 'make W=1' shows a warning for an empty macro:

security/apparmor/label.c: In function '__label_update':
security/apparmor/label.c:2096:59: error: suggest braces around empty body in an 'else' statement [-Werror=empty-body]
 2096 |                 AA_BUG(labels_ns(label) != labels_ns(new));

Change the macro definition to use no_printk(), which improves
format string checking and avoids the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: John Johansen <john.johansen@canonical.com>