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: NWan Jiabing <wanjiabing@vivo.com>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Acked-by: NJohn Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20211012023231.19911-1-wanjiabing@vivo.com

tools/testing/selftests/bpf/cgroup_helpers.c

@@ -11,7 +11,6 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <ftw.h>
-#include <unistd.h>
 
 #include "cgroup_helpers.h"