mm/page_alloc.c: remove duplicate inclusion of page_ext.h

Link: http://lkml.kernel.org/r/20170202011942.1609-1-standby24x7@gmail.com


Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>