net: export time stamp utility function for Ethernet MAC drivers

The network stack provides the function, skb_clone_tx_timestamp().
Ethernet MAC drivers can call this via the transmit time stamping
hook, skb_tx_timestamp(). This commit exports the clone function so
that drivers using it can be compiled as modules.
Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>

net/core/timestamping.c

@@ -68,6 +68,7 @@ void skb_clone_tx_timestamp(struct sk_buff *skb)
 		break;
 	}
 }
+EXPORT_SYMBOL_GPL(skb_clone_tx_timestamp);
 
 void skb_complete_tx_timestamp(struct sk_buff *skb,
 			       struct skb_shared_hwtstamps *hwtstamps)