LICENSES: Add Linux syscall note exception

Copied from the Linux kernel COPYING file.

Add the required tags for reference and tooling.

Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Philippe Ombredanne <[e-mail]>
Reviewed-by: Kate Stewart <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

LICENSES/exceptions/Linux-syscall-note

···
+SPDX-Exception-Identifier: Linux-syscall-note
+SPDX-URL: https://spdx.org/licenses/Linux-syscall-note.html
+SPDX-Licenses: GPL-2.0, GPL-2.0+, GPL-1.0+, LGPL-2.0, LGPL-2.0+, LGPL-2.1, LGPL-2.1+
+Usage-Guide:
+  This exception is used together with one of the above SPDX-Licenses
+  to mark user space API (uapi) header files so they can be included
+  into non GPL compliant user space application code.
+  To use this exception add it with the keyword WITH to one of the
+  identifiers in the SPDX-Licenses tag:
+    SPDX-License-Identifier: <SPDX-License> WITH Linux-syscall-note
+License-Text:
+
+   NOTE! This copyright does *not* cover user programs that use kernel
+ services by normal system calls - this is merely considered normal use
+ of the kernel, and does *not* fall under the heading of "derived work".
+ Also note that the GPL below is copyrighted by the Free Software
+ Foundation, but the instance of code that it refers to (the Linux
+ kernel) is copyrighted by me and others who actually wrote it.
+
+ Also note that the only valid version of the GPL as far as the kernel
+ is concerned is _this_ particular version of the license (ie v2, not
+ v2.2 or v3.x or whatever), unless explicitly otherwise stated.
+
+			Linus Torvalds
+