x86/xen: fix non-ANSI declaration of xen_has_pv_devices()

xen_has_pv_devices() has no parameters, so use the normal void
parameter convention to make it match the prototype in the header file
include/xen/platform_pci.h.
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NDavid Vrabel <david.vrabel@citrix.com>

arch/x86/xen/platform-pci-unplug.c

@@ -68,7 +68,7 @@ static int check_platform_magic(void)
 	return 0;
 }
 
-bool xen_has_pv_devices()
+bool xen_has_pv_devices(void)
 {
 	if (!xen_domain())
 		return false;