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: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
diff --git a/arch/x86/xen/platform-pci-unplug.c b/arch/x86/xen/platform-pci-unplug.c
index a826171..9586ff3 100644
--- a/arch/x86/xen/platform-pci-unplug.c
+++ b/arch/x86/xen/platform-pci-unplug.c
@@ -68,7 +68,7 @@
 	return 0;
 }
 
-bool xen_has_pv_devices()
+bool xen_has_pv_devices(void)
 {
 	if (!xen_domain())
 		return false;