reuse: Add dep5 lines for gnulib and libcharset

Since they are copylibs, we don’t want to diverge from upstream by adding SPDX lines to all the files, so add them to the dep5 file instead.

gnulib discussed adding them upstream a couple of years ago but the discussion seemed to peter out: https://lists.gnu.org/archive/html/bug-gnulib/2021-06/msg00004.html.

Signed-off-by: Philip Withnall pwithnall@gnome.org