x86/platform/uv: Mark uv_min_hub_revision_id static

This variable is only used inside x2apic_uv_x and not even declared
in a header.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Not-acked-by: Dimitri Sivanich <sivanich@hpe.com>
Cc: Russ Anderson <rja@hpe.com>
Link: https://lkml.kernel.org/r/20200504171527.2845224-8-hch@lst.de