[tools] Fix fix-includes script

Newer iwyu versions will exit with an error code in case include
issues have been found.

tools/fix-includes

@@ -56,7 +56,9 @@ trap cleanup exit
 cd "${BUILDDIR}"
 
 echo "Running IWYU"
-ninja iwyu > "${TMPFILE}"
+set +e
+ninja iwyu |tee "${TMPFILE}"
+set -e
 
 echo "Running fix_includes.py"
 fix_includes.py < "${TMPFILE}"