Syndication: Relicense notice

[Christophe Giboudeaux]

Hi,

While porting the syndication files to use the SPDX license format, some files 
added by Frerich Raabe which originally lived in 'librss' (> 15 years ago) 
were marked as problematic. 

dataretriever.{cpp,h} and loader.{cpp,h} only have a partial license header 
pointing to a COPYING file which I can't find anywhere.

According to the relicensecheck script, all developers who pushed changes to 
these files allow to relicense files to LGPL-2.0-or-later.
Unless someone has a valid objection, I'll update the license header for these 
files in a couple days.

Christophe