skbuff.h: fix timestamps kernel-doc

Fix skbuff.h kernel-doc for timestamps: must include "struct" keyword,
otherwise there are kernel-doc errors:

Error(linux-next-20090227//include/linux/skbuff.h:161): cannot understand prototype: 'struct skb_shared_hwtstamps '
Error(linux-next-20090227//include/linux/skbuff.h:177): cannot understand prototype: 'union skb_shared_tx '

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>