COPYING: state that all contributions really are covered by this file

Explicitly state that all contributions to the kernel source tree
really are covered under this COPYING file in case someone thought
otherwise.  Lawyers love to be pedantic, even more so than software
engineers at times, and this sentence makes them sleep easier.
Reviewed-by: NThomas Gleixner <tglx@linutronix.de>
Acked-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200206154800.GA3754085@kroah.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

COPYING

@@ -16,3 +16,5 @@ In addition, other licenses may also apply. Please see:
 	Documentation/process/license-rules.rst
 
 for more details.
+
+All contributions to the Linux Kernel are subject to this COPYING file.