gpio: sim: Declare gpio_sim_hog_config_item_ops static

Compiler is not happy:

  warning: symbol 'gpio_sim_hog_config_item_ops' was not declared. Should it be static?

Fixes: cb8c474e ("gpio: sim: new testing module")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>