[ARM] Kirkwood: __init requires linux/init.h

Include linux/init.h for __init to fix this error:

CC [M]  drivers/net/wireless/wl12xx/boot.o
In file included from arch/arm/mach-kirkwood/include/mach/gpio.h:13,
                 from arch/arm/include/asm/gpio.h:5,
                 from include/linux/gpio.h:7,
                 from drivers/net/wireless/wl12xx/boot.c:24:
arch/arm/plat-orion/include/plat/gpio.h:32: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘orion_gpio_init’
make[6]: *** [drivers/net/wireless/wl12xx/boot.o] Error 1
make[5]: *** [drivers/net/wireless/wl12xx] Error 2

Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Nicolas Pitre <nico@marvell.com>