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>