kbuild: Remove -Waggregate-return from scripts/Makefile.extrawarn

kbuild: Remove -Waggregate-return from scripts/Makefile.extrawarn

It makes little sense to pass -Waggregate-return these days since large
part of the linux kernel rely on returning struct(s). For instance:

  ../include/linux/timekeeping.h: In function 'show_uptime':
  ../include/linux/ktime.h:91:34: error: function call has aggregate value [-Werror=aggregate-return]
   #define ktime_to_timespec64(kt)  ns_to_timespec64((kt))
                                    ^~~~~~~~~~~~~~~~~~~~~~
  ../include/linux/timekeeping.h:166:8: note: in expansion of macro 'ktime_to_timespec64'
    *ts = ktime_to_timespec64(ktime_get_coarse_boottime());

Remove this warning from W=2 completely.

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>