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

LICENSES/other/X11

+Valid-License-Identifier: X11
+SPDX-URL: https://spdx.org/licenses/X11.html
+Usage-Guide:
+  To use the X11 put the following SPDX tag/value pair into a comment
+  according to the placement guidelines in the licensing rules
+  documentation:
+    SPDX-License-Identifier: X11
+License-Text:
+
+
+X11 License
+
+Copyright (C) 1996 X Consortium
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of this software and associated documentation files (the "Software"),
+to deal in the Software without restriction, including without limitation
+the rights to use, copy, modify, merge, publish, distribute, sublicense,
+and/or sell copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+X CONSORTIUM BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+Except as contained in this notice, the name of the X Consortium shall not
+be used in advertising or otherwise to promote the sale, use or other
+dealings in this Software without prior written authorization from the X
+Consortium.
+
+X Window System is a trademark of X Consortium, Inc.