debian/control: Depend on the hotdoc-0.8 branch

Newer hotdoc (0.10) introduced several breaking changes which cause the
documentation to be misbuilt, so let's explicitly depend on the old version.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

debian/control

@@ -7,8 +7,8 @@ Build-Depends:
  autotools-dev,
  debhelper (>= 9),
  dh-autoreconf,
- hotdoc (>= 0.8),
- hotdoc-c-extension,
+ hotdoc-0.8 (>= 0.8),
+ hotdoc-c-extension-0.8,
  libchamplain-0.12-dev,
  libclutter-gst-3.0-dev,
  libgirepository1.0-dev,