Ribbon compilation fixes for OS X.

Forward declare wxWindow and wxDC classes in wx/ribbon/art.h to avoid errors
in (PCH-less?) buildbot builds.

Also include header containing declarations of private Mac functions in
implementation file (closes #11203).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61949 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/ribbon/art.h

@@ -19,6 +19,9 @@
 #include <wx/brush.h>
 #include <wx/pen.h>
 
+class WXDLLIMPEXP_FWD_CORE wxDC;
+class WXDLLIMPEXP_FWD_CORE wxWindow;
+
 enum wxRibbonArtSetting
 {
     wxRIBBON_ART_TAB_SEPARATION_SIZE,