net: dsa: include gpio consumer header file

After the introduction of the switch gpio reset API, I'm getting
build errors in configurations that disable CONFIG_GPIOLIB:

net/dsa/dsa.c:783:16: error: implicit declaration of function 'gpio_to_desc' [-Werror=implicit-function-declaration]

The reason is that linux/gpio/consumer.h is not automatically
included without gpiolib support. This adds an explicit #include
statement to make it compile in all configurations. The reset
functionality will not work without gpiolib, which is what you
get when disabling the feature.

As far as I can tell, gpiolib is supported on all architectures
on which you can have DSA at the moment.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: cc30c163 ("net: dsa: Add support for a switch reset gpio")
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>