gpio: consumer.h: Move forward declarations outside #ifdef

Make sure that the forward declared structs in gpio/consumer.h are also visible
on the else branch of the CONFIG_GPIOLIB #ifdef.

Fixes the following warnings and their associated errors when CONFIG_GPIOLIB is
not selected:
	include/linux/gpio/consumer.h:67:14: warning: 'struct device' declared inside parameter list
	include/linux/gpio/consumer.h:67:14: warning: its scope is only this definition or declaration, which is probably not what you want
	[...]

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>