iio: ak8975: fix AK09911 dependencies

ak8975 depends on I2C and GPIOLIB, so any symbols that selects
ak8975 must have the same dependency, or we get build errors:

drivers/iio/magnetometer/ak8975.c: In function 'ak8975_who_i_am':
drivers/iio/magnetometer/ak8975.c:393:2: error: implicit declaration of function 'i2c_smbus_read_i2c_block_data' [-Werror=implicit-function-declaration]
  ret = i2c_smbus_read_i2c_block_data(client, AK09912_REG_WIA1,
  ^
drivers/iio/magnetometer/ak8975.c: In function 'ak8975_set_mode':
drivers/iio/magnetometer/ak8975.c:431:2: error: implicit declaration of function 'i2c_smbus_write_byte_data' [-Werror=implicit-function-declaration]
  ret = i2c_smbus_write_byte_data(data->client,

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 57e73a42 ("iio: ak8975: add ak09911 and ak09912 support")
Signed-off-by: Jonathan Cameron <jic23@kernel.org>