CI: treat documentation warnings as errors

We do not want to merge documentation that produces Sphinx warnings.

scripts/kernel-doc uses environment variable KDOC_WERROR to determine
if warnings should be treated as errors.

Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index b193fee..e6c6ab3 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -171,7 +171,7 @@
     - virtualenv -p /usr/bin/python3 /tmp/venvhtml
     - . /tmp/venvhtml/bin/activate
     - pip install -r doc/sphinx/requirements.txt
-    - make htmldocs
+    - make htmldocs KDOC_WERROR=1
     - make infodocs
 
 # some statistics about the code base