samples: bpf: Add the omitted xdp samples to .gitignore

There are recently added xdp samples (xdp_redirect_map_multi and
xdpsock_ctrl_proc) which are not managed by .gitignore.

This commit adds these files to .gitignore.

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210727041056.23455-2-claudiajkang@gmail.com