spi: bcm2835: depends GPIOLIB

I got following error on CONFIG_GPIOLIB=n.

drivers/spi/spi-bcm2835.c: In function 'chip_match_name':
drivers/spi/spi-bcm2835.c:356:21: error: dereferencing pointer to incomplete type 'struct gpio_chip'
  return !strcmp(chip->label, data);
                     ^
drivers/spi/spi-bcm2835.c: In function 'bcm2835_spi_setup':
drivers/spi/spi-bcm2835.c:382:9: error: implicit declaration of function 'gpiochip_find' [-Werror=implicit-function-declaration]
  chip = gpiochip_find("pinctrl-bcm2835", chip_match_name);
         ^
drivers/spi/spi-bcm2835.c:382:7: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
  chip = gpiochip_find("pinctrl-bcm2835", chip_match_name);
       ^
drivers/spi/spi-bcm2835.c: In function 'chip_match_name':
drivers/spi/spi-bcm2835.c:357:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^

Signed-off-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Mark Brown <broonie@kernel.org>