Input: cpcap-pwrbutton - replace GPLv2 boilerplate with SPDX

Replace the GPLv2 boilerplate text with a nice and short
SPDX header.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Link: https://lore.kernel.org/r/20230705212231.631525-1-sre@kernel.org


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>