lkdtm/bugs: Switch from 1-element array to flexible array

The testing for ARRAY_BOUNDS just wants an uninstrumented array,
and the proper flexible array definition is fine for that.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Bill Wendling <morbo@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>