xen/xenbus: Fix compile warning.

We were missing the 'void' on the parameter arguments.
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xenbus/xenbus_xs.c

@@ -625,7 +625,7 @@ static struct xenbus_watch *find_watch(const char *token)
  * so if we are running on anything older than 4 do not attempt to read
  * control/platform-feature-xs_reset_watches.
  */
-static bool xen_strict_xenbus_quirk()
+static bool xen_strict_xenbus_quirk(void)
 {
 	uint32_t eax, ebx, ecx, edx, base;