gpio: guard gpiochip_irqchip_add_domain() with GPIOLIB_IRQCHIP

The current code doesn't check if GPIOLIB_IRQCHIP is enabled, which results in
a compilation error when trying to build gpio-regmap if CONFIG_GPIOLIB_IRQCHIP
isn't enabled.

Fixes: 6a45b0e2 ("gpiolib: Introduce gpiochip_irqchip_add_domain()")
Suggested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Michael Walle <michael@walle.cc>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20210324081923.20379-2-noltari@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>