systemd: Reident the expression file

The indenting is a bit weird to follow, especially at the end of the
file (right brace without indent, but the opening brace is indented by
two spaces).

No functional change and I've verified this by building it with this
change and without and both lead to the same store paths.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @fpletz, @edolstra