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

@@ -9,9 +9,9 @@ Build-Depends:
  debhelper (>= 10),
  dh-apparmor,
  gobject-introspection,
- hotdoc (>= 0.8),
- hotdoc-c-extension,
- hotdoc-dbus-extension,
+ hotdoc-0.8 (>= 0.8),
+ hotdoc-c-extension-0.8,
+ hotdoc-dbus-extension-0.8,
  libgirepository1.0-dev,
  libglib2.0-dev (>= 2.58.2),
  libpolkit-gobject-1-dev,