x86: xen: move cpu_up functions out of ifdef

Three newly introduced functions are not defined when CONFIG_XEN_PVHVM is
disabled, but are still being used:

arch/x86/xen/enlighten.c:141:12: warning: ‘xen_cpu_up_prepare’ used but never defined
arch/x86/xen/enlighten.c:142:12: warning: ‘xen_cpu_up_online’ used but never defined
arch/x86/xen/enlighten.c:143:12: warning: ‘xen_cpu_dead’ used but never defined

Fixes: 4d737042

 ("xen/x86: Convert to hotplug state machine")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>