Fix distribution of the applet service files sources when building from
an in-process-configured build. This fixes `make distcheck` and
tarballs in builds configured with `--with-in-process-applets=all` (or
actually any in-process applet).
The issue was that the .service.in file was referenced indirectly in
EXTRA_DIST, and the variable listed there was only filled in a
conditional. Automake is not able to resolve this and see it still
should distribute the reference of that variable, even if set
conditionally. Fix this by always setting the variable properly, even
if it's only used for out-of-process builds.