fib: make netdev_switch_fib_ipv4_abort in header file static inline

When building without CONFIG_NET_SWITCHDEV,
netdev_switch_fib_ipv4_abort is defined in the header file. It must
be static inline to avoid build failure at link time.

Fixes: 8e05fd71

 ("fib: hook IPv4 fib for hardware offload")

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>