decnet: Move rtm_dn_policy to dn_route to make it available if !CONFIG_DECNET_ROUTER

Otherwise build fails with CONFIG_DECNET && !CONFIG_DECNET_ROUTER

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>