Fail the build if it updates any of the files in the git repository

There are still some pregenerated files left in the git
repository (cleaned up during `make maintainer-clean`) and we currently
don't notice if any of those needs to be updated in the git repository
because we ignore changes in the repository done during the build.

This commit adds a safeguard that fails the build job if the contents of
the git repository gets modified during the build.

.gitlab-ci.yml

@@ -201,6 +201,7 @@ stages:
     - make -j${BUILD_PARALLEL_JOBS:-1} -k all V=1
     - test -z "${RUN_MAKE_INSTALL}" || make install
     - test -z "${RUN_MAKE_INSTALL}" || sh util/check-make-install
+    - if test "$(git status --porcelain | grep -Ev '\?\?' | wc -l)" -gt "0"; then git status --short; exit 1; fi
   needs:
     - job: autoreconf
       artifacts: true