includecheck fix: include/linux, page_cgroup.h

fix the following 'make includecheck' warning:

  include/linux/page_cgroup.h: linux/swap.h is included more than once.

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>