Include linux/gpio/consumer.h instead of linux/gpio.h, as is said in the
latter file.

This was reported by kernel test bot when compiling for s390.

  drivers/pci/controller/pci-aardvark.c:350:2: error: implicit declaration of function 'gpiod_set_value_cansleep' [-Werror,-Wimplicit-function-declaration]
  drivers/pci/controller/pci-aardvark.c:1074:21: error: implicit declaration of function 'devm_gpiod_get_from_of_node' [-Werror,-Wimplicit-function-declaration]
  drivers/pci/controller/pci-aardvark.c:1076:14: error: use of undeclared identifier 'GPIOD_OUT_LOW'

Link: https://lore.kernel.org/r/202006211118.LxtENQfl%25lkp@intel.com
Link: https://lore.kernel.org/r/20200907111038.5811-2-pali@kernel.org
Fixes: 5169a985 ("PCI: aardvark: Issue PERST via GPIO")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>