Stop using undefined reject function in check_dsc_files

Stop using undefined reject function in check_dsc_files

When there's an unknown file in a source package, the function spits
this:

  ,---
  Processing raised an exception: global name 'reject' is not defined.
  Traceback (most recent call last):
    File "/srv/ftp-master.debian.org/dak/dak/daklib/archive.py", line 966, in check
      chk().check(self)
    File "/srv/ftp-master.debian.org/dak/dak/daklib/checks.py", line 497, in check
      rejects = utils.check_dsc_files(dsc_fn, control, source.files.keys())
    File "/srv/ftp-master.debian.org/dak/dak/daklib/utils.py", line 343, in check_dsc_files
      reject("%s: unexpected source file '%s'" % (dsc_filename, f))
  NameError: global name 'reject' is not defined
  `---

Just set the reject message and break from the loop.