Add missing required header to wx/generic/private/grid.h

Make this header self-sufficient, instead of requiring wx/headerctrl.h
to be included before including it.

Remove the now unnecessary wx/headerctrl.h inclusion from
src/generic/grideditors.cpp.

include/wx/generic/private/grid.h

@@ -15,6 +15,8 @@
 
 #if wxUSE_GRID
 
+#include "wx/headerctrl.h"
+
 // Internally used (and hence intentionally not exported) event telling wxGrid
 // to hide the currently shown editor.
 wxDECLARE_EVENT( wxEVT_GRID_HIDE_EDITOR, wxCommandEvent );