apparmor: fix zero-length compiler warning in AA_BUG()

Uses of AA_BUG() without a message can result in the compiler warning

  warning: zero-length gnu_printf format string [-Wformat-zero-length]

Fix this with a pragma for now. A larger rework of AA_BUG() will
follow.

Signed-off-by: John Johansen <john.johansen@canonical.com>