tools: Sync uapi/linux/if_link.h

Sync uapi/linux/if_link.h into tools to avoid out of sync warnings during
libbpf build.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200113073143.1779940-2-andriin@fb.com