hw/s390x: Add missing 'static' attribute

Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/s390x/event-facility.c

@@ -32,7 +32,7 @@ struct SCLPEventFacility {
     unsigned int receive_mask;
 };
 
-SCLPEvent cpu_hotplug;
+static SCLPEvent cpu_hotplug;
 
 /* return true if any child has event pending set */
 static bool event_pending(SCLPEventFacility *ef)