export tcp_nuke_addr to build ipv6 as module

Otherwise we run into following build error:
-----8<-----
  ...
  CC [M]  lib/zlib_deflate/deftree.o
  CC [M]  lib/zlib_deflate/deflate_syms.o
  LD [M]  lib/zlib_deflate/zlib_deflate.o
  Building modules, stage 2.
  MODPOST 46 modules
ERROR: "tcp_nuke_addr" [net/ipv6/ipv6.ko] undefined!
make[2]: *** [__modpost] Error 1
make[1]: *** [modules] Error 2
make: *** [sub-make] Error 2
-----8<-----

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>