fib: Fix undef compile warning

net/core/fib_rules.c:26:7: warning: "CONFIG_IP_MULTIPLE_TABLES" is not defined, evaluates to 0 [-Wundef]
 #elif CONFIG_IP_MULTIPLE_TABLES
       ^~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 8b66a6fd

 ("fib: fix another fib_rules_ops indirect call wrapper problem")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-By: Brian Vazquez <brianvv@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>