tools: sync uapi/linux/if_link.h header

This file has not been updated for a while.

Sync it before BIG TCP patch series.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20211122184810.769159-1-eric.dumazet@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>