skbuff.h: fix kernel-doc warning for headers_end

@@ -557,7 +557,9 @@ struct sk_buff {
 	/* fields enclosed in headers_start/headers_end are copied
 	 * using a single memcpy() in __copy_skb_header()
 	 */
+	/* private: */
 	__u32			headers_start[0];
+	/* public: */
 
 /* if you move pkt_type around you also must adapt those constants */
 #ifdef __BIG_ENDIAN_BITFIELD

@@ -642,7 +644,9 @@ struct sk_buff {
 	__u16			network_header;
 	__u16			mac_header;
 
+	/* private: */
 	__u32			headers_end[0];
+	/* public: */
 
 	/* These elements must be at the end, see alloc_skb() for details.  */
 	sk_buff_data_t		tail;