cgroup: remove duplicated include from cgroup.c

Remove duplicated include.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index dd26e1b..861995c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -61,7 +61,6 @@
 #include <linux/cpuset.h>
 #include <linux/proc_ns.h>
 #include <linux/nsproxy.h>
-#include <linux/proc_ns.h>
 #include <net/sock.h>
 
 /*