Sync if_link.h into tools/ and get rid of annoying libbpf Makefile warning.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191021033902.3856966-2-andriin@fb.com