Fix ppc64el build failure, by not redefining AT_VECTOR_SIZE_ARCH

This fixes the ppc64el build failure
  CC       criu/arch/ppc64/sigframe.o
In file included from criu/arch/ppc64/sigframe.c:5:0:
criu/arch/ppc64/include/asm/types.h:32:0: error: "AT_VECTOR_SIZE_ARCH" redefined [-Werror]
 #define AT_VECTOR_SIZE_ARCH 6

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>