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

install-prereqs.sh

diff --git a/install-prereqs.sh b/install-prereqs.sh
index e5763a4..9c178e0 100755
--- a/install-prereqs.sh
+++ b/install-prereqs.sh
@@ -54,6 +54,7 @@
     libglib2.0-dev-bin
     libgtk2.0-0
     libpixman-1-dev
+    libpython3.8
     libsqlite3-dev
     libssl-dev
     libtinfo-dev