Bug 1253617 - Fix non-unified build bustage in OverscrollHandoffState.cpp r=BenWa

MozReview-Commit-ID: 9QkpsxPZsQS

gfx/layers/apz/src/OverscrollHandoffState.h

@@ -15,6 +15,9 @@
 #include "Units.h"            // for ScreenPoint
 
 namespace mozilla {
+
+class InputData;
+
 namespace layers {
 
 class AsyncPanZoomController;