samples, bpf: Add xsk_fwd test file to .gitignore

This commit adds xsk_fwd test file to .gitignore which is newly added
to samples/bpf.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200904063434.24963-2-danieltimlee@gmail.com