x86/platform/uv: Unexport sn_coherency_id

sn_coherency_id is only used by x2apic_uv_x.c, and uv_sysfs.c, both
of which can't be modular.

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/20200504171527.2845224-4-hch@lst.de