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>

net/core/timestamping.c

@@ -122,6 +122,7 @@ bool skb_defer_rx_timestamp(struct sk_buff *skb)
 
 	return false;
 }
+EXPORT_SYMBOL_GPL(skb_defer_rx_timestamp);
 
 void __init skb_timestamping_init(void)
 {