gpiolib: unexport gpiochip_get_desc()

This function has been deprecated for some time and is now only used
within the GPIOLIB core. Remove it from the public header and unexport
it as all current users are linked against the compilation unit where
it is defined.

Link: https://lore.kernel.org/r/20240625073815.12376-1-brgl@bgdev.pl


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>