sysfs, kernfs: remove duplicated include from file.c

Remove duplicated include.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/file.c

@@ -13,7 +13,6 @@
 #include <linux/slab.h>
 #include <linux/poll.h>
 #include <linux/pagemap.h>
-#include <linux/poll.h>
 #include <linux/sched.h>
 
 #include "kernfs-internal.h"