leds: triggers: oneshot: fix module license specification

The header of this file fixes the license to GPL 2 only without the
option to use later version. So use the string "GPL v2" that is to be
used in this case.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Fabio Baltieri <fabio.baltieri@gmail.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>