LICENSES: Add X11 license

Add the full text of the X11 to the kernel tree.  It was copied directly
from:

   https://spdx.org/licenses/X11.html#licenseText



Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>