sparc64: Export __node_distance.

Some drivers reference it via node_distance(), for example the
NVME host driver core.

ERROR: "__node_distance" [drivers/nvme/host/nvme-core.ko] undefined!
make[1]: *** [scripts/Makefile.modpost:92: __modpost] Error 1
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc/mm/init_64.c

@@ -1383,6 +1383,7 @@ int __node_distance(int from, int to)
 	}
 	return numa_latency[from][to];
 }
+EXPORT_SYMBOL(__node_distance);
 
 static int __init find_best_numa_node_for_mlgroup(struct mdesc_mlgroup *grp)
 {