selftests/bpf: add uprobe_multi test binary to .gitignore

It seems like it was forgotten to add uprobe_multi binary to .gitignore.
Fix this trivial omission.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230824000016.2658017-1-andrii@kernel.org


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>