doc: gsg: add libpython3.8-dev
doc: gsg: add libpython3.8-dev

not all distributions install python3.8 libraries as a part of the
python3 package. Specifying the python3.8 library solves this problem.

Signed-off-by: Zach Hudson <aa6c353b937b55a65f84172832b7a3bc5f60e803@phytec.com>

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