docs: fix out-of-tree doc checks with gtk-doc 1.17
docs: fix out-of-tree doc checks with gtk-doc 1.17

gtk-doc 1.17 runs in the builddir, not the sourcedir. So these files end
up in the builddir, which is where we're running.

Unfortunately this breaks doc checks with gtk-doc ≤ 1.16, which I can't
find the strength to care about.

(This is akin to 3ab0d76 in tp-glib.)

freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control