Tell Automake about _gen/svc-gtk-doc.h
Tell Automake about _gen/svc-gtk-doc.h

When the codegen was updated, this started being built, but not cleaned
up, so distcheck was broken.

freedesktop-unofficial-mirror/telepathy__telepathy-idle,freedesktop-unofficial-mirror/telepathy__telepathy-idle,freedesktop-unofficial-mirror/telepathy__telepathy-idle