extensions/Makefile.am: ensure there's a rule for _gen/cli-body.h
extensions/Makefile.am: ensure there's a rule for _gen/cli-body.h

This fixes parallel builds, at least on my laptop.

freedesktop-unofficial-mirror/telepathy__telepathy-logger,freedesktop-unofficial-mirror/telepathy__telepathy-logger,freedesktop-unofficial-mirror/telepathy__telepathy-logger