net: lib: Fix missing dns subdir include
net: lib: Fix missing dns subdir include

The dns directory was not included when building with
MDNS_RESONDER or LLMR_RESPONDER configs.

Signed-off-by: Loic Poulain <7dbc6498a90ff1ada42fc224430b4f763baa5c5c@linaro.org>

zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr