COMP: Added export macros for events in charts.

This issue became apparent on Windows while testing the modular build
system as the 2D rendering is now separate from the charts classes
there.

Change-Id: Ic83146945b1af113c3f9674369abaace4b8833da

Charts/vtkContextKeyEvent.h

@@ -27,7 +27,7 @@
 
 class vtkRenderWindowInteractor;
 
-class vtkContextKeyEvent
+class VTK_CHARTS_EXPORT vtkContextKeyEvent
 {
 public:
   vtkContextKeyEvent();

Charts/vtkContextMouseEvent.h

@@ -26,7 +26,7 @@
 
 class vtkRenderWindowInteractor;
 
-class vtkContextMouseEvent
+class VTK_CHARTS_EXPORT vtkContextMouseEvent
 {
 public:
   // Description: