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>

scripts/Makefile.extrawarn

@@ -11,6 +11,8 @@
 # are not supported by all versions of the compiler
 # ==========================================================================
 
+KBUILD_CFLAGS += $(call cc-disable-warning, packed-not-aligned)
+
 ifeq ("$(origin W)", "command line")
   export KBUILD_ENABLE_EXTRA_GCC_CHECKS := $(W)
 endif
@@ -26,6 +28,7 @@ warning-1 += -Wold-style-definition
 warning-1 += $(call cc-option, -Wmissing-include-dirs)
 warning-1 += $(call cc-option, -Wunused-but-set-variable)
 warning-1 += $(call cc-option, -Wunused-const-variable)
+warning-1 += $(call cc-option, -Wpacked-not-aligned)
 warning-1 += $(call cc-disable-warning, missing-field-initializers)
 warning-1 += $(call cc-disable-warning, sign-compare)