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: David S. Miller <davem@davemloft.net>