res_counter: remove include of cgroup.h from res_counter.h

It's not needed at all.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/res_counter.h

@@ -13,7 +13,7 @@
  * info about what this counter is.
  */
 
-#include <linux/cgroup.h>
+#include <linux/spinlock.h>
 
 /*
  * The core object. the cgroup that wishes to account for some