x86/xen: Link platform-pci-unplug.o only if CONFIG_XEN_PVHVM

Instead of using one large #ifdef CONFIG_XEN_PVHVM in
arch/x86/xen/platform-pci-unplug.c add the object file depending on
CONFIG_XEN_PVHVM being set.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: xen-devel@lists.xenproject.org
Cc: virtualization@lists.linux-foundation.org
Cc: akataria@vmware.com
Cc: rusty@rustcorp.com.au
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/20180828074026.820-4-jgross@suse.com