ssb: Drop legacy header include

The SSB header only uses the legacy <linux/gpio.h> header to get
struct gpio_chip so inluce <linux/gpio/driver.h> which is the right
include to deal with gpio_chip.

Cc: Michael Buesch <m@bues.ch>
Cc: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210821235800.138817-1-linus.walleij@linaro.org