Follow-up to bug 1073081 - Fix the non-unified build bustage

dom/ipc/PreallocatedProcessManager.cpp

@@ -7,6 +7,7 @@
 #include "mozilla/PreallocatedProcessManager.h"
 #include "mozilla/ClearOnShutdown.h"
 #include "mozilla/Preferences.h"
+#include "mozilla/unused.h"
 #include "mozilla/dom/ContentParent.h"
 #include "mozilla/dom/ScriptSettings.h"
 #include "nsIPropertyBag2.h"