xen: dbgp: Fix warning when CONFIG_PCI is not enabled.

I saw this on ARM:
linux/drivers/xen/dbgp.c:11:23: warning: unused variable 'ctrlr' [-Wunused-variable]
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/dbgp.c

@@ -8,7 +8,9 @@
 
 static int xen_dbgp_op(struct usb_hcd *hcd, int op)
 {
+#ifdef CONFIG_PCI
 	const struct device *ctrlr = hcd_to_bus(hcd)->controller;
+#endif
 	struct physdev_dbgp_op dbgp;
 
 	if (!xen_initial_domain())