Include wx/defs.h from all our public headers, including wx/imaglist.h.

Without including wx/defs.h first, symbols such as __WXMSW__ are not defined
if this header is the first wxWidgets header to be included.

Closes #11718.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63553 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/imaglist.h

@@ -12,6 +12,8 @@
 #ifndef _WX_IMAGLIST_H_BASE_
 #define _WX_IMAGLIST_H_BASE_
 
+#include "wx/defs.h"
+
 /*
  * wxImageList is used for wxListCtrl, wxTreeCtrl. These controls refer to
  * images for their items by an index into an image list.