selftests/bpf: Remove duplicated include in cgroup_helpers

Fix following checkincludes.pl warning:
./scripts/checkincludes.pl tools/testing/selftests/bpf/cgroup_helpers.c
tools/testing/selftests/bpf/cgroup_helpers.c: unistd.h is included more
than once.

Signed-off-by: Wan Jiabing <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: John Fastabend <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/cgroup_helpers.c

···
 #include <fcntl.h>
 #include <unistd.h>
 #include <ftw.h>
-#include <unistd.h>
 
 #include "cgroup_helpers.h"