selftests/bpf: eliminate warning of get_cgroup_id_from_path()

selftests/bpf: eliminate warning of get_cgroup_id_from_path()

The output goes like this if I make samples/bpf:
...warning: no previous prototype for ‘get_cgroup_id_from_path’...

Make this function static could solve the warning problem since
no one outside of the file calls it.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240406144613.4434-1-kerneljasonxing@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>