modules: openthread: Fix building with picolibc
modules: openthread: Fix building with picolibc

The OpenThread module should not explicitly link with libc in case of
minimal libc or picolibc.

Signed-off-by: Pieter De Gendt <b5b50e3ea5dc0014327690ec78e146f3b45ee870@basalte.be>

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