x86/xen: make CONFIG_XEN depend on CONFIG_X86_LOCAL_APIC

Since commit feb44f1f (x86/xen:
Provide a "Xen PV" APIC driver to support >255 VCPUs) Xen guests need
a full APIC driver and thus should depend on X86_LOCAL_APIC.

This fixes an i386 build failure with !SMP && !CONFIG_X86_UP_APIC by
disabling Xen support in this configuration.

Users needing Xen support in a non-SMP i386 kernel will need to enable
CONFIG_X86_UP_APIC.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Cc: <stable@vger.kernel.org>