MIPS: Loongson3: Fix __node_distances undefined error

export the __node_distances symbol in the loongson3 numa code to fix the
build error:

  Building modules, stage 2.
  MODPOST 221 modules
ERROR: "__node_distances" [drivers/block/nvme.ko] undefined!
scripts/Makefile.modpost:90: recipe for target '__modpost' failed

when building the kernel with:
 CONFIG_CPU_LOONGSON3=y
 CONFIG_NUMA=y
 CONFIG_BLK_DEV_NVME=m

Signed-off-by: James Cowgill <James.Cowgill@imgtec.com>
Cc: <stable@vger.kernel.org> # v3.17+
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Huacai Chen <chenhc@lemote.com>
Cc: linux-mips@linux-mips.org
Cc: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Patchwork: https://patchwork.linux-mips.org/patch/8444/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>