ipv4: remove fib_update_nh_saddrs() declaration.

This patch removes the fib_update_nh_saddrs() declaration from
include/net/ip_fib.h, as the fib_update_nh_saddrs() method was removed in
coomit 436c3b66 ("ipv4: Invalidate nexthop cache nh_saddr more correctly").

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>