gitlab-ci: add linux-3.17 tag to the sphinx build

This build uses Qt libraries in a Fedora container. This library is
tagged with a note which says it requires Linux 3.17. Some of or CI
runners do not have this kernel on the host which causes glibc to refuse
to load it. Add a tag to indicate that we need a newer kernel on the
runner.

.gitlab-ci.yml

@@ -113,6 +113,7 @@ build:fedora31-sphinx:
         - build
         - docker
         - linux
+        - linux-3.17 # Needed to be able to load Fedora's Qt libraries.
 
     script:
         - .gitlab/ci/sccache.sh