nds32: Fix building error of crypto/xor.c by adding xor.h

When I compiled with allmodconfig, it caused this building failed.
crypto/xor.c:25:21: fatal error: asm/xor.h: No such file or directory
 #include <asm/xor.h>
                     ^
compilation terminated.

Signed-off-by: Greentime Hu <greentime@andestech.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>