Make opensuse platform job non-voting again

The non-voting flag was accidentally dropped in [0].

[0] Ib4416dc2f5e003fd770f5240a8f78213c56af8e6

Change-Id: If9519f1ac9afd66553e1c1410fdc16369f166b98
diff --git a/.zuul.yaml b/.zuul.yaml
index f78f3f5..4142d46 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -587,6 +587,7 @@
     parent: tempest-full-py3
     description: openSUSE 15.x platform test
     nodeset: devstack-single-node-opensuse-15
+    voting: false
 
 - job:
     name: devstack-platform-focal