gpiolib: of: add forward declaration for struct device_node

gpiolib: of: add forward declaration for struct device_node

Commit 08a149c40bdb ("gpiolib: Clean up headers") added a forward
declaration for struct device.

Later, commit 07445ae1c263 ("gpiolib: of: change of_find_gpio() to
accept device node") changed the function signature to accept a struct
device_node instead of a struct device.

Replace forward declaration of struct device with struct device_node to
match the updated function signature.

Signed-off-by: Andrei Lalaev <andrey.lalaev@gmail.com>
Link: https://lore.kernel.org/r/20250721155737.261990-1-andrey.lalaev@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>