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: NIngo Molnar <mingo@elte.hu>

arch/x86/xen/enlighten.c

@@ -35,6 +35,7 @@
 #include <xen/page.h>
 
 #include <asm/paravirt.h>
+#include <asm/apic.h>
 #include <asm/page.h>
 #include <asm/xen/hypercall.h>
 #include <asm/xen/hypervisor.h>