cabal: fix Data-files syntax

36b1d769 removed the last entry from
this list but did not remove the trailing comma from the previous line.

When I am king, all file formats will support trailing commas in lists.
1 job for master in 24 minutes and 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #3991335
flatpak-build

00:24:06