get_maintainer.pl: fix source tree detection

get_maintainer.pl always fails with following message:
./scripts/get_maintainer.pl: The current directory does not appear to be a linux kernel source tree.

This was caused by commit:

commit 548b310c
Author: Masahiro Yamada <yamada.m@jp.panasonic.com>
Date:   Thu Oct 30 15:50:15 2014 +0900

    Remove the CREDITS file

    This file is not maintained these days.

    We use MAINTAINERS for the maintainership of the supported boards.
    For dead boards, we have some clues in doc/README.scrapyard and
    also imperishable history in git-log.

Remove CREDITS from source tree detection to fix this.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>