selftests/bpf: Add xdp_redirect_multi into .gitignore

When xdp_redirect_multi test binary was added recently, it wasn't added to
.gitignore. Fix that.

Fixes: d2329247 ("selftests/bpf: Add xdp_redirect_multi test")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210603004026.2698513-5-andrii@kernel.org