fixing branch compile error (bug 333079) r=marria

extensions/metrics/src/nsMetricsEventItem.h

@@ -42,6 +42,7 @@
 #include "nsIMetricsService.h"
 #include "nsCOMArray.h"
 #include "nsCOMPtr.h"
+#include "nsString.h"
 
 class nsIPropertyBag;