[IPV4]: Declarations cleanup in ip_fib.h.

Two small issues fixed:
- fib_select_multipath is exported from fib_semantics.c rather than from
  fib_frontend.c. So, move the declaration below appropriate comment.
- struct rt_entry declaration is not used. Drop it.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>