hg: shenandoah/jdk11: Fix post-merge build problem: Shenandoah should be disabled with Zero

[shade at redhat.com]

Changeset: 3e6a9b28a2fb
Author:    shade
Date:      2019-03-06 18:06 +0100
URL:       http://hg.openjdk.java.net/shenandoah/jdk11/rev/3e6a9b28a2fb

Fix post-merge build problem: Shenandoah should be disabled with Zero

! make/autoconf/hotspot.m4