x86/xen: Make xen_reservation_lock static

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Fixes: f030aade ("x86/xen: Move pv specific parts of arch/x86/xen/mmu.c to mmu_pv.c")
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: xen-devel@lists.xenproject.org
Cc: boris.ostrovsky@oracle.com
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/20180905053634.13648-1-jgross@suse.com