all: Add LICENSE (and fix SPDX deprecated header)

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>

kdumpst-load.header

@@ -1,7 +1,7 @@
 #!/bin/bash
 set -o pipefail  #  Didn't set -u on purpose, since users could pass empty $1
 #
-#  SPDX-License-Identifier: LGPL-2.1+
+#  SPDX-License-Identifier: LGPL-2.1-or-later
 #
 #  Copyright (c) 2021 Valve.
 #  Maintainer: Guilherme G. Piccoli <gpiccoli@igalia.com>