net: export the receive time stamping hook for non-NAPI drivers

Ethernet MAC drivers based on phylib (but not using NAPI) can
enable hardware time stamping in phy devices by calling netif_rx()
conditionally based on a call to skb_defer_rx_timestamp().

This commit exports that function so that drivers calling it may
be compiled as modules.

Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>