Input: gpio-keys - make gpio_keys_device_driver static

This fixes the sparse warning

	symbol 'gpio_keys_device_driver' was not declared. Should it be static?

Signed-off-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>