samples: lwm2m_client: Fix building on native_posix
samples: lwm2m_client: Fix building on native_posix

Sample code uses GPIO api's but wasn't enabling CONFIG_GPIO.  So
on native_posix this would cause the sample to not build.  Add
CONFIG_GPIO=y to prj.conf to fix this.

Fixes #48306

Signed-off-by: Kumar Gala <383ef5577c6e1178b93f59ec8d0936f76d2a98c4@kernel.org>

galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr