cgroup: add missing ns_common include

Add the missing include of the ns_common header.

Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

include/linux/cgroup_namespace.h

@@ -2,6 +2,8 @@
 #ifndef _LINUX_CGROUP_NAMESPACE_H
 #define _LINUX_CGROUP_NAMESPACE_H
 
+#include <linux/ns_common.h>
+
 struct cgroup_namespace {
 	struct ns_common	ns;
 	struct user_namespace	*user_ns;