s390/lib: add missing include

Include <asm/xor.h> into arch/s390/lib/xor.c to expose xor_block_xc
declaration and avoid the following sparse warning:
arch/s390/lib/xor.c:128:27: warning: symbol 'xor_block_xc' was not declared. Should it be static?

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>