if: restore token ring ARP type to header

Recent removal of Token Ring breaks the build of iproute2.

Even though Token Ring support is gone from the kernel, it is worth
keeping the the definition of the TR ARP type to avoid breaking
userspace programs that use this file.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>