libgsmhr/fetch_sources.py: fix source code downloading

For some reason, the 3GPP server started to reject the source
code download request:

  urllib2.HTTPError: HTTP Error 403: Forbidden

Adding both 'User-Agent' and 'Accept' headers solves the problem.

Change-Id: I8afd39c85dd3e450c5355888ab6edcf4c81f8b87
1 file changed