LICENSES: Add the LGPL-2.1 license

Add the full text of the LGPL 2.1 license to the kernel tree.  It was
copied directly from:

   https://spdx.org/licenses/LGPL-2.1.html#licenseText

Add the required tags for reference and tooling.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Philippe Ombredanne <pombredanne@nexb.com>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>