gnu: darktable: Update to 3.0.2.

* gnu/packages/photo.scm (darktable): Update to 3.0.2.
[arguments]: Compile with clang.  Wrap GSETTINGS_SCHEMAS_DIR.
[license]: Add license information for Rawspeed.
[native-inputs]: Add clang, desktop-file-utils, glib, gobject-instrospection,
llvm, and po4a.  Reorder alphabetically.
[inputs]: Add colord-gtk, cups, graphicsmagick, gsettings-desktop-schemas,
iso-codes, libsecret, lua, openjpeg, and osm-gps-map.  Reorder alphabetically.

Signed-off-by: Marius Bakke <marius@gnu.org>