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>

arch/s390/lib/xor.c

@@ -9,6 +9,7 @@
 #include <linux/types.h>
 #include <linux/export.h>
 #include <linux/raid/xor.h>
+#include <asm/xor.h>
 
 static void xor_xc_2(unsigned long bytes, unsigned long *p1, unsigned long *p2)
 {