x86/jailhouse: Mark jailhouse_x2apic_available() as __init

.. as it is only called at early bootup stage.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: jailhouse-dev@googlegroups.com
Link: https://lkml.kernel.org/r/1561539289-29180-1-git-send-email-zhenzhong.duan@oracle.com