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

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: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200206154800.GA3754085@kroah.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>