Bug 1513911 - Part 2 - fixed a missing include; r=mccr8

Depends on D18995

Differential Revision: https://phabricator.services.mozilla.com/D21411

--HG--
extra : moz-landing-system : lando

ipc/ipdl/test/cxx/IPDLUnitTestUtils.h

@@ -2,6 +2,8 @@
 #ifndef mozilla__ipdltest_IPDLUnitTestUtils
 #define mozilla__ipdltest_IPDLUnitTestUtils 1
 
+#include "ipc/IPCMessageUtils.h"
+
 namespace mozilla {
 namespace _ipdltest {