Correct permissions for `md5sums`.

According to `lintian` the file `md5sums` should have the permissions 0644. Without this patch the following is returned from a `lintian` run:

```sh
E: PACKAGE_NAME: control-file-has-bad-permissions md5sums 0664 != 0644
```

More details can be found [here](http://lintian.debian.org/tags/control-file-has-bad-permissions.html) and [here](http://www.debian.org/doc/debian-policy/ch-files.html#s-permissions-owners).