xen/pciback: remove duplicated #include

Remove duplicated #include('s) in
  drivers/xen/xen-pciback/xenbus.c
Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xen-pciback/xenbus.c

@@ -11,7 +11,6 @@
 #include <xen/xenbus.h>
 #include <xen/events.h>
 #include <asm/xen/pci.h>
-#include <linux/workqueue.h>
 #include "pciback.h"
 
 #define	DRV_NAME	"xen-pciback"