gtk-doc: fix api-index warnings

Fix gtk-doc warnings such as:
html/SpiceChannel.html:538: warning: no link for: "api-index-0.20" -> (0.20).

Unfortunately, the generated file is still missing.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
3 jobs for build-sys in 5 minutes and 19 seconds (queued for 1 second)
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Status Job ID Name Coverage
  Test
passed #84212
makecheck

00:05:00

passed #84213
makecheck_simple

00:04:09

passed #84214
make-win

00:05:19