gpio: ath79: fix implicit assumption module.h is present

gpio: ath79: fix implicit assumption module.h is present

The Kconfig for this file is:

drivers/gpio/Kconfig:config GPIO_ATH79
drivers/gpio/Kconfig: tristate "Atheros AR71XX/AR724X/AR913X GPIO support"

...but however it does not include module.h -- it in turn gets it from
another header (gpio/driver.h) and we'd like to replace that with a
forward delcaration of "struct module;" but if we do, this file will
fail to compile.

So we fix this first to avoid putting build failures into the bisect
commit history.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>