Input: gpio_keys - add missing include to gpio_keys.h

gpio_keys.h uses 'bool' - type which is defined in linux/types.h.
Include this header.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>