gpio: nomadik: wrap a local variable in a necessary ifdef

gpio: nomadik: wrap a local variable in a necessary ifdef

The 'desc' local variable in nmk_gpio_dbg_show_one() is now only used
with CONFIG_PINCTRL_NOMADIK enabled so wrap its declaration with an
appropriate ifdef.

Fixes: ddeb66d2cb10 ("gpio: nomadik: don't print out global GPIO numbers in debugfs callbacks")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202509032125.nXBcPuaf-lkp@intel.com/
Link: https://lore.kernel.org/r/20250903131903.95100-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>