Add util.h as header for util.c.

Previously we were abusing primary_with_secondary.h.

Change-Id: I07b91a4e96d1c27bee4f5250724f119c126f6770
diff --git a/test/vmapi/primary_with_secondaries/interrupts.c b/test/vmapi/primary_with_secondaries/interrupts.c
index 7791c64..655ca54 100644
--- a/test/vmapi/primary_with_secondaries/interrupts.c
+++ b/test/vmapi/primary_with_secondaries/interrupts.c
@@ -22,6 +22,7 @@
 
 #include "hftest.h"
 #include "primary_with_secondary.h"
+#include "util.h"
 
 /**
  * Send a message to the interruptible VM, which will interrupt itself to send a