Some of the files in `malcontent-control` are GPL-2.0+.

Use the latest SPDX identifiers in the machine-readable data in
meson.build, but use the deprecated ‘+’ form in the README since it’s a
little more human friendly.

See https://spdx.org/licenses/

.

Signed-off-by: Philip Withnall <withnall@endlessm.com>