Thanks to the heads-up from Foxboron (on Arch Linux IRC channel)
add hereby a LICENSE file to the project that matches the SPDX
headers. While at it, fix the SPDX short identifier on headers,
that was deprecated (as per [0]).

The license file was a pure copy obtained today from [1].

[0] https://spdx.org/licenses/LGPL-2.1+.html
[1] https://www.gnu.org/licenses/old-licenses/lgpl-2.1.txt

Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>