pkg-config: export systemd{system,user}generatordir and catalogdir

We export the location of a bunch of directories this way,
so it makes sense to add those three. Especially catalogdir
is something that we want people to add things to.

Note on the naming: the first two are tied closely to systemd
itself, so I prefixed them with "systemd". The third one is
rather more generic, so no prefix.

https://bugs.freedesktop.org/show_bug.cgi?id=67635