tools: show diff for headerguards check on failure

    fixes #8737

dist/tools/ci/build_and_test.sh

@@ -92,7 +92,7 @@ then
         run ./dist/tools/pr_check/pr_check.sh ${CI_BASE_BRANCH}
         run ./dist/tools/coccinelle/check.sh
         run ./dist/tools/flake8/check.sh
-        QUIET=1 run ./dist/tools/headerguards/check.sh
+        run ./dist/tools/headerguards/check.sh
         exit $RESULT
     fi