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>

arch/nds32/include/asm/Kbuild

@@ -49,6 +49,7 @@ generic-y += switch_to.h
 generic-y += timex.h
 generic-y += topology.h
 generic-y += trace_clock.h
+generic-y += xor.h
 generic-y += unaligned.h
 generic-y += user.h
 generic-y += vga.h