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 <[email protected]>
Reviewed-by: Philippe Ombredanne <[email protected]>
Reviewed-by: Kate Stewart <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>