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

tools/testing/selftests/bpf/cgroup_helpers.c

@@ -429,7 +429,7 @@ int create_and_get_cgroup(const char *relative_path)
  * which is an invalid cgroup id.
  * If there is a failure, it prints the error to stderr.
  */
-unsigned long long get_cgroup_id_from_path(const char *cgroup_workdir)
+static unsigned long long get_cgroup_id_from_path(const char *cgroup_workdir)
 {
 	int dirfd, err, flags, mount_id, fhsize;
 	union {