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>

include/linux/skbuff.h

@@ -135,8 +135,7 @@ struct skb_frag_struct {
 #define HAVE_HW_TIME_STAMP
 
 /**
- * skb_shared_hwtstamps - hardware time stamps
- *
+ * struct skb_shared_hwtstamps - hardware time stamps
  * @hwtstamp:	hardware time stamp transformed into duration
  *		since arbitrary point in time
  * @syststamp:	hwtstamp transformed to system time base
@@ -164,8 +163,7 @@ struct skb_shared_hwtstamps {
 };
 
 /**
- * skb_shared_tx - instructions for time stamping of outgoing packets
- *
+ * struct skb_shared_tx - instructions for time stamping of outgoing packets
  * @hardware:		generate hardware time stamp
  * @software:		generate software time stamp
  * @in_progress:	device driver is going to provide