Fix drace CTF for the rib_head.

33cb3cb2e321 introduced an rib_head structure field under the
FIB_ALGO define. This may be problematic for the CTF, as some
of the files including route_var.h do not have fib_algo
defined.

Make dtrace happy by making the field unconditional.

Suggested by: markj

(cherry picked from commit bc5ef45aec3fa8acf2dd3408cebd207317543a8b)