scripts/checkincludes.pl: add exit message for no duplicates found

If no duplicates found, inform user.

Link: http://lkml.kernel.org/r/1486391275-2843-1-git-send-email-thrust73@gmail.com
Signed-off-by: Cheah Kok Cheong <thrust73@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>