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>

Author: anakryiko

tools/testing/selftests/bpf/.gitignore

@@ -44,6 +44,7 @@ test_cpp
 /bench
 /veristat
 /sign-file
+/uprobe_multi
 *.ko
 *.tmp
 xskxceiver