check-cocci fails in WARNING is found on stderr

As the implicit cast check print "WARNING: ..." on stderr, add a pattern
to make sure that check-cocci would fails if such warning is found on
stderr. This is generic (not specific like the existing "parse error")
so it should be able to support future Coccinelle spatch warnings.

util/check-cocci.sh

@@ -23,7 +23,7 @@ run_spatch() {
   echo "Applying semantic patch $spatch..."
   spatch --jobs "${TEST_PARALLEL_JOBS:-1}" --sp-file "$spatch" --use-gitgrep --dir "." --include-headers $spatchargs >>"$patch" 2>cocci.stderr
   cat cocci.stderr
-  if grep -q -e "parse error" -e "EXN: Failure" cocci.stderr; then
+  if grep -q -e "parse error" -e "EXN: Failure" -e "WARNING" cocci.stderr; then
     ret=1
   fi
   if [ "$(wc <"$patch" -l)" -gt "0" ]; then