fix the following 'make includecheck' warning:

  include/linux/page_cgroup.h: linux/swap.h is included more than once.
Signed-off-by: NJaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: NPaul Menage <menage@google.com>