tools: Sync if_xdp.h uapi tooling header

Sync if_xdp.h uapi header to remove following warning:

Warning: Kernel ABI header at 'tools/include/uapi/linux/if_xdp.h'
differs from latest version at 'include/uapi/linux/if_xdp.h'

Fixes: 48eb03dd2630 ("xsk: Add TX timestamp and TX checksum offload support")
Signed-off-by: Vishal Chourasia <[e-mail]>
Signed-off-by: Song Yoong Siang <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/include/uapi/linux/if_xdp.h

···
 	((1ULL << XSK_UNALIGNED_BUF_OFFSET_SHIFT) - 1)
 
 /* Request transmit timestamp. Upon completion, put it into tx_timestamp
- * field of union xsk_tx_metadata.
+ * field of struct xsk_tx_metadata.
  */
 #define XDP_TXMD_FLAGS_TIMESTAMP		(1 << 0)
 
 /* Request transmit checksum offload. Checksum start position and offset
- * are communicated via csum_start and csum_offset fields of union
+ * are communicated via csum_start and csum_offset fields of struct
  * xsk_tx_metadata.
  */
 #define XDP_TXMD_FLAGS_CHECKSUM			(1 << 1)