docs: cdomain.py: get rid of a warning since version 1.8

There's a new warning about a deprecation function. Add a
logic at cdomain.py to avoid that.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sphinx/cdomain.py

@@ -48,7 +48,10 @@ major, minor, patch = sphinx.version_info[:3]
 
 def setup(app):
 
-    app.override_domain(CDomain)
+    if (major == 1 and minor < 8):
+        app.override_domain(CDomain)
+    else:
+        app.add_domain(CDomain, override=True)
 
     return dict(
         version = __version__,