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>

criu/arch/ppc64/include/asm/types.h

@@ -29,7 +29,9 @@ static inline uint64_t encode_pointer(void *p) { return (uint64_t)p; }
  *	include/linux/mm_types.h
  */
 #define AT_VECTOR_SIZE_BASE	20
+#if !defined AT_VECTOR_SIZE_ARCH
 #define AT_VECTOR_SIZE_ARCH	6
+#endif
 #define AT_VECTOR_SIZE		(2*(AT_VECTOR_SIZE_ARCH + AT_VECTOR_SIZE_BASE + 1))
 
 typedef uint64_t auxv_t;