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>