doc: portability/posix: Indicate presence of strtok_r
doc: portability/posix: Indicate presence of strtok_r

Commit 0637595e added strtok_r implementation but missed updating this
document.

Signed-off-by: Siddharth Chandrasekaran <cf3591b207d7ad6b41008948dad8c0c751800981@embedjournal.com>

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