util/readline: Fix lints for readline_handle_byte

util/readline: Fix lints for readline_handle_byte

While they do not give warnings under our current buildsystem
configuration, my clang's language server daemon was complaining about
missing default: labels in switch statements.

While at it, add /* fallthrough */ annotations where appropriate.

This is a purely style and not functional change.

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <16f745ac7f5fef74498709ffd98857e76edff6aa.1718265822.git.manos.pitsidianakis@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>