meson: Avoid using relative paths to po directory

Replace the relative path to the po directory with a toplevel definition
of po_dir and use that instead.

!507
4 jobs for !507 with benzea/systemd-user-units in 8 minutes and 11 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Review
passed #331904
check_commit_log

00:00:31

 
  Build
passed #331905
build

00:06:42

 
  Test
passed #331906
test

00:00:56

passed #331907
test-pot

00:00:56