gnu: Add 'file-name' fields for github source tarballs without a name.

* gnu/packages/aarddict.scm (aarddict), gnu/packages/algebra.scm (arb),
  gnu/packages/audio.scm (jack-2, lvtk),
  gnu/packages/bioinformatics.scm (bedops, bedtools, bowtie),
  gnu/packages/game-development.scm (tiled),
  gnu/packages/games.scm (minetest-data, minetest, retroarch),
  gnu/packages/jrnl.scm (jrnl), gnu/packages/kde.scm (qjson),
  gnu/packages/libevent.scm (libuv), gnu/packages/linux.scm (pflask),
  gnu/packages/mail.scm (offlineimap, libetpan),
  gnu/packages/maths.scm (arpack-ng), gnu/packages/ninja.scm (ninja),
  gnu/packages/nutrition.scm (gourmet),
  gnu/packages/python.scm (python-cairocffi), gnu/packages/rdf.scm (lrdf),
  gnu/packages/ruby.scm (ruby-i18n), gnu/packages/sxiv.scm (sxiv): Add
  'file-name' field to origin.