added po4a dependency to bootstrap.sh

Problem: my build was failing after setting up the buildroot via bootstrap.sh on ubuntu 20.04

Solution: i've added the missing dependency to it.

Changes introduced by this pull request:

• added po4a to ubuntu/debian install block

State: ready

Error:

po4a/update-po: The program 'po4a' was not found. po4a/update-po: Translated man pages were not generated. cook - xz - error: failed to fetch: failed to run "bash" "-ex": exited with status exit code: 1 ./repo.sh failed.make: *** [mk/filesystem.mk:18: build/filesystem.bin] Error 1