scripts: Add libpython3.8 to prereqs

RISC-V GDB is built with python3.8 so it can be compatible with
Ubuntu18.04, Ubuntu20.04, and Debian testing. To run GDB, the system
needs to have libpython3.8 installed.

Change-Id: I6cc89ef9dc098c607e781586d7321e3995ac6a69
1 file changed