Do not include linux/backing-dev.h twice

Don't include linux/backing-dev.h twice in mm/filemap.c, it's pointless.
Signed-off-by: NJesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -28,7 +28,6 @@
 #include <linux/backing-dev.h>
 #include <linux/pagevec.h>
 #include <linux/blkdev.h>
-#include <linux/backing-dev.h>
 #include <linux/security.h>
 #include <linux/syscalls.h>
 #include <linux/cpuset.h>