x86/jailhouse: Hide x2apic code when CONFIG_X86_X2APIC=n

x2apic_phys is not available when CONFIG_X86_X2APIC=n and the code is not
optimized out resulting in a build fail:

jailhouse.c: In function ‘jailhouse_get_smp_config’:
jailhouse.c:73:3: error: ‘x2apic_phys’ undeclared (first use in this function)

Fixes: 11c8dc41 ("x86/jailhouse: Enable APIC and SMP support")
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: jailhouse-dev@googlegroups.com