Kbuild: suppress packed-not-aligned warning for default setting only

gcc-8 reports many -Wpacked-not-aligned warnings. The below are some
examples.

./include/linux/ceph/msgr.h:67:1: warning: alignment 1 of 'struct
ceph_entity_addr' is less than 8 [-Wpacked-not-aligned]
 } __attribute__ ((packed));

./include/linux/ceph/msgr.h:67:1: warning: alignment 1 of 'struct
ceph_entity_addr' is less than 8 [-Wpacked-not-aligned]
 } __attribute__ ((packed));

./include/linux/ceph/msgr.h:67:1: warning: alignment 1 of 'struct
ceph_entity_addr' is less than 8 [-Wpacked-not-aligned]
 } __attribute__ ((packed));

This patch suppresses this kind of warnings for default setting.

Signed-off-by: Xiongfeng Wang <xiongfeng.wang@linaro.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>