getting-started: Add missing system dependencies for tests

Without this dependency, the pifpaf tests won't execute. They will
fail with a cryptic error.

That cryptic error message is currently fixed by pifpaf's author. It's
pending in a PR and should soon land.

Related P341
Related https://github.com/jd/pifpaf/issues/108
Related https://github.com/jd/pifpaf/pull/119

docs/getting-started.rst

@@ -54,7 +54,8 @@ package manager. On Debian/Ubuntu-based distributions::
   sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'
   sudo apt update
   sudo apt install python3 python3-venv libsvn-dev postgresql-10 nodejs \
-                   libsystemd-dev libpython3-dev dia postgresql-autodoc
+                   libsystemd-dev libpython3-dev dia postgresql-autodoc \
+                   postgresql-server-dev-all
 
 Postgres
 ~~~~~~~~