x86, xen: fix apic_ops build on UP

fix:

 arch/x86/xen/enlighten.c:615: error: variable ‘xen_basic_apic_ops’ has initializer but incomplete type
 arch/x86/xen/enlighten.c:616: error: unknown field ‘read’ specified in initializer
 [...]

Signed-off-by: Ingo Molnar <mingo@elte.hu>