Remove CPScrollView build warning

CPRulerView.j did not import CPScrollView.
A direct import would create a cycle, since CPScrollView.j already imports CPRulerView.j.
Added a forward declaration instead.

AppKit/CPTextView/CPRulerView.j

@@ -36,6 +36,7 @@ CPRulerOrientationHorizontal = 0,
 CPRulerOrientationVertical = 1
 
 @class CPRulerView;
+@class CPScrollView;
 
 
 // MARK: - CPRulerMarker (Interactive Handles with Dynamic Alignment Icons)