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 <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Martin KaFai Lau <[email protected]>

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 {